Publication List

2021

  1. Unraveling the Activity of Iron Carbide Clusters Embedded in Silica for Thermocatalytic Conversion of Methane [Abstract]

    Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidative conversion of gaseous methane with high selectivity. The activated catalyst was proposed to be FeC2 cluster embedded in silica. Using a combination of density-functional theoretic methods and micro-kinetic modeling, we show that under the same reaction conditions (1223 K , 1 atm) FeC2 sites convert to FeC3 and the latter is instead responsible for the observed activity. We investigate the detailed mechanism of conversion of methane to methyl radical and hydrogen on FeC3@SiO2 under different conditions of methane partial pressure. We find that methyl radical evolution is the rate-determining step for the overall conversion. Our calculations also indicate that the conversion of embedded FeC3 to FeC4 competes with methyl radical evolution from the active catalyst. However, due to the higher stability of FeC3 sites, we anticipate that formation of higher carbides can be inhibited by controlling the hydrogen partial pressure.

    Gopal K. Dixit, Manish Kumar, Ankita Katiyar, Antonius P. J. Jansen, Alexander P. van Bavel, Ravi Agrawal, Prathamesh M. Shenai, Varadharajan Srinivasan
    Catal. Sci. Technol., 2021, Accepted Manuscript (2021)
  2. Direct and indirect role of Fe doping in NiOOH monolayer for water oxidation catalysis [Abstract]

    The oxygen evolution reaction (OER) activity of pristine NiOOH is enhanced by doping with Fe. However, the precise role of Fe is still being debated. Here, we use first-principles DFT+U approach to study three different types of active sites: one on pristine and the other two on Fe-doped NiOOH monolayers to account for the direct and indirect roles of Fe. To compare the activity of the active sites, we consider two mechanisms of OER based on the source of O-O bond formation. Our results show that the mechanism involving the coupling of lattice oxygen is generally more favorable than water nucleophilic attack on lattice oxygen. On doping with Fe, the overpotential of NiOOH is reduced by 0.29 V in excellent agreement with experimental findings. Doping Fe in NiOOH results in different potential determining steps (PDS) in the two mechanisms, whereas pristine NiOOH has the same PDS regardless of the mechanism.

    Manish Kumar, Simone Piccinin, and Varadharajan Srinivasan
    10.33774/chemrxiv-2021-kthjg-v2 (2021)
  3. Pressure induced topochemical polymerization of solid acrylamide facilitated by anisotropic response of hydrogen bond network [Abstract]

    The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crys- talline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. While our calculations are able to reproduce experimentally measured pressure dependent spectroscopic features, the atomistic analysis confirms the absence of polymerization in acrylamide at the experimentally estimated pres- sure of 17-20 GPa. Instead, we predict polymerization at a higher pressure of ∼23 GPa at 0 K albeit through large enthalpy barriers. Interestingly, we find that the two-dimensional hydrogen bond network in acrylamide templates a topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both N-H...O and C- H...O bonds. Using a simple model for thermal effects, we also show that at 300K higher pressures significantly accelerate the transformation into polymer by lowering the barrier. Thus, application of pressure offers an alternative route for topochemical polymerization when higher temperatures are undesirable.

    Sayan Maity, Abhijeet S. Gangan, Ashwini Anshu, Rashid Rafeek V. Valappil, Brahmananda Chakraborty, Lavanya M. Ramaniah, and Varadharajan Srinivasan
    Phys. Chem. Chem. Phys., 2021, 23, 9448 - 9556
  4. Dynamics of Anthracene Excimer Formation within a Water-soluble Nanocavity at Room Temperature [Abstract]

    Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host–guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host–guest complexation by NMR, probing the host–guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer–excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

    Aritra Das, Ashwini Danao, Shubhojit Banerjee, A. Mohan Raj, Gaurav Sharma; Rajeev Prabhakar, Varadharjan Srinivasan, Vaidhyanathan Ramamurthy, and Pratik Sen
    J. Am. Chem. Soc. 2021, 143, 2025–2036.

2020

  1. Unraveling the Activity of Iron Carbide Clusters Embedded in Silica for Thermocatalytic Conversion of Methane [Abstract]

    Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidative conversion of gaseous methane with high selectivity. The activated catalyst was proposed to be FeC2 cluster embedded in silica. Using a combination of density-functional theoretic methods and micro-kinetic modeling, we show that under the same reaction conditions (1223 K , 1 atm) FeC2 sites convert to FeC3 and the latter is instead responsible for the observed activity. We investigate the detailed mechanism of conversion of methane to methyl radical and hydrogen on FeC3@SiO2 under different conditions of methane partial pressure. We find that methyl radical evolution is the rate-determining step for the overall conversion. Our calculations also indicate that the conversion of embedded FeC3 to FeC4 competes with methyl radical evolution from the active catalyst. However, due to the higher stability of FeC3 sites, we anticipate that formation of higher carbides can be inhibited by controlling the hydrogen partial pressure.

    Gopal K. Dixit, Manish Kumar, Ankita Katiyar, Antonius P. J. Jansen, Alexander P. van Bavel, Ravi Agrawal, Prathamesh M. Shenai, Varadharajan Srinivasan
    10.26434/chemrxiv.13347107.v1 (2020)
  2. Selective functionalization at N2-position of guanine in oligonucleotides via reductive amination [Abstract]

    Chemo- and site-specific modifications in oligonucleotides have wide applicability as mechanistic probes in chemical biology. However, methods that label specific sites in nucleic acids are scarce, especially for labeling DNA/RNA from biological or enzymatic sources rather than synthetic ones. Here we have employed a classical reaction, reductive amination, to selectively functionalize the N2-amine of guanosine and 2’-deoxyguanosine monophosphate (GMP/dGMP). This method specifically modifies guanine in DNA and RNA oligonucleotides, while leaving the other nucleobases unaffected. Using this approach, we have successfully incorporated a reactive handle chemoselectively into nucleic acids for further functionalization and downstream applications.

    Bapurao A. Bhoge, Purnima Mala, Jo S. Kurian, Varadharajan Srinivasan, and Ishu Saraogi
    Chem. Commun., 2020, 56, 13832-13835
  3. Pressure Induced Topochemical Polymerization of Solid Acryalmide Facilitated by Anisotropic Response of Hydrogen Bond Network [Abstract]

    The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) based study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. Our calculations predict a polymerization pressure of 94 GPa, in contrast to experimental estimates of 17 GPa, while being able to reproduce the experimentally measured pressure dependent spectroscopic features. Interestingly, we find that the two-dimensional hydrogen bond network templates a topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both N-H...O and C-H...O bonds.

    Sayan Maiti, Abhijeet Sadashiv Gangan, Ashwini Anshu, Rashid Rafeek V. Valappil, Brahmananda Chakraborty, Lavanya M. Ramaniah, and Varadharajan Srinivasan
    10.26434/chemrxiv.12519881 (2020)

2019

  1. Exploring the structural , electronic and magnetic properties of cation ordered 3d-5d double perovskite Bi2FeReO6 and Bi2FeIrO6 thin-films from first-principles [Abstract]

    We report a first-principles study of Bi-based 3d−5d ordered double-perovskite oxides (A2BB'O6) with a 3d atom (Fe) at the B site and 5d atoms (Re, Ir) at the B' site while keeping highly polarizable ions (Bi3+) at the A site. We find that, under coherent heteroepitaxy, Bi2FeReO6 exhibits a strain-driven antiferromagnetic insulator to ferrimagnetic semimetal transition, while Bi2FeIrO6 shows a correlation driven ferromagnetic insulator to ferrimagnetic half-metal transition with calculated magnetic moments of 5 and 3μ/f.u., respectively. These properties along with the low band gaps in the insulating phases make the compounds appealing for spintronics applications. Furthermore, in Bi2FeIrO6, the conduction and valence states are localized on different transition metal sublattices implying more efficient electron-hole separation upon photoexcitation, a desirable feature for photovoltaic applications.

    Paresh Chandra Rout, and Varadharajan Srinivasan
    Phys. Rev. B. 100, 245136 (2019)
  2. Emergence of a multiferroic half-metallic phase in Bi2FeCrO6 through interplay of hole doping and epitaxial strain [Abstract]

    Epitaxial strain has been shown to drive structural phase transitions along with novel functionalities in perovskite-based thin films. Aliovalent doping at the A site can drive an insulator-to-metal and magnetic transitions in perovskites along with a variety of interesting structural and electronic phenomena. Using first-principles calculations, we predict the formation of a multiferroic half-metallic phase with a large magnetic moment in the double perovskite, Bi2FeCrO6 by coupling epitaxial strain with A-site hole doping. We also demonstrate that epitaxial strain can be used to manipulate the hole states created by doping to induce half-metal to insulator, antipolar to polar, antiferromagnetic to ferromagnetic, orbital ordering and charge ordering transitions. Our work also suggests that hole doping under strain could lead to mitigation of issues related to antisite defects and lowered magnetization in thin films of the material

    Paresh Chandra Rout, and Varadharajan Srinivasan
    Phys. Rev. Lett. 123, 107201 (2019)

2018

  1. Epitaxial strain control of hole-doping induced phases in a multiferroic Mott insulator Bi2FeCrO6
    Paresh Chandra Rout, and Varadharajan Srinivasan
    arXiv:1807.01649 (2018)
  2. Phase diagram for the Harper model of the honeycomb lattice [Abstract]

    The Harper equation arising out of a tight-binding model of electrons on a honeycomb lattice subject to a uniform magnetic field perpendicular to the plane is studied. Contrasting and complementary approaches involving von Neumann entropy, fidelity, fidelity susceptibility, and multifractal analysis are employed to characterize the phase diagram. Remarkably even in the absence of the quasi-periodic on-site potential term, the Hamiltonian allows for a metal-insulator transition. The phase diagram consists of three phases: two metallic phases and an insulating phase. A variant model where next nearest neighbor hopping is included, exhibits a mobility edge and does not allow for a simple single phase diagram characterizing all the eigenstates.

    Geo Jose, Rajesh Malla, Varadharajan Srinivasan, Auditya Sharma, and Suhas Gangadharaiah
    J. Phys.: Condens. Matter 30, 385603 (2018)
  3. Giant Ferrimagnetism and Polarization in Mixed Metal Perovskite Metal-organic Frameworks [Abstract]

    Perovskite metal-organic frameworks (MOFs) have recently emerged as potential candidates for multiferroicity. However, the compounds synthesized so far possess only weak ferromagnetism and low polarization. Additionally, the very low magnetic transition temperatures (Tc) also pose a challenge to the application of the materials. We have computationally designed a mixed metal perovskite MOF— [C(NH2)3][(Cu0.5Mn0.5)(HCOO)3]—that is predicted to have magnetization two orders of magnitude larger than its parent ([C(NH2)3][Cu(HCOO)3]), a significantly larger polarization (9.9μC/cm2), and an enhanced Tc of up to 56 K, unprecedented in perovskite MOFs. A detailed study of the magnetic interactions revealed a mechanism leading to the large moments as well as the increase in the Tc. Mixing a non-Jahn-Teller ion (Mn2+) into a Jahn-Teller host (Cu2+) leads to competing lattice distortions which are directly responsible for the enhanced polarization. The MOF is thermodynamically stable as evidenced by the computed enthalpy of formation and can likely be synthesized. Our work represents a first step towards rational design of multiferroic perovskite MOFs through the largely unexplored mixed metal approach.

    Paresh Chandra Rout, and Varadharajan Srinivasan
    Phys. Rev. Materials 2, 014407 (2018)

2016

  1. Origin of lowered magnetic moments in epitaxially strained thin films of multiferroic Bi2FeCrO6 [Abstract]

    We have investigated the effect of epitaxial strain on the magnetic properties and B-site cation ordering in multiferroic Bi2FeCrO6 (001) thin films using a density-functional theory approach. We find that in thin films with rock-salt ordering of Fe and Cr the ground state is characterized by C-type antiferromagnetic (AFM) order. This is in contrast to the bulk form of the material, which was predicted to be a ferrimagnet with G-type AFM order. Furthermore, the cation-ordered thin films undergo a transition with epitaxial strain from C- to A-type AFM order. Other magnetic orders appear as thermally accessible excited states. We also find that B-site cation-disordered structures are more stable in coherent epitaxial strains, thereby explaining the lowered magnetic moments observed in these samples at room temperature. Strain varies both the sign and strength of the Fe-Cr superexchange coupling, resulting in a very interesting phase diagram for Bi2FeCrO6 thin films.

    Paresh Chandra Rout, Aditya Putatunda, and Varadharajan Srinivasan
    Phys. Rev. B 93, 104415 (2016)

2015

  1. Electronic Topological Transitions In Cd At High Pressures [Abstract]

    Pressure-induced changes in the Fermi surface of Cd up to 40 GPa are studied using highly accurate density-functional theory calculations. The topology of Fermi surface changes at pressures of 2, 8, 12 , 18 and 28 GPa indicating electronic topological transitions (ETTs). Structural parameters, compressibility data and elastic constant reveal anomalies across these ETTs. The computed equation of state at 300 K is in excellent agreement with the experimental data. In view of the highly controversial nature of these ETTs the present studies are aimed at motivating the experimentalists for their direct detection at high pressures.

    Varadharajan Srinivasan, B. K. Godwal, Jeffrey C. Grossman, and Raymond Jeanloz
    arXiv:1511.01989 (2015)
  2. Engineering the optical response of a-Se thin films by employing morphological disorder [Abstract]

    In this article, we experimentally demonstrate for the first time that photobleaching (PB) can be induced in morphologically disordered a-Se thin film, an observation which is opposite of the previously well-known photodarkening (PD) effects in morphologically ordered films. Further, the optical response of the film shows many fold increase with increase in control beam intensity. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. Our results demonstrate an efficient and yet simple new method to engineer the optical response of photosensitive thin films. We envision that this process can open up many avenues in optical field-enhanced absorption-based technologies.

    Rituraj Sharma, Deepak Kumar, Varadharajan Srinivasan, H. Jain, and K. V. Adarsh
    Opt. Express 23, 14085 (2015)

2014

  1. Exploring the Potential of Fulvalene Dimetals as Platforms for Molecular Solar Thermal Energy Storage: Computations, Syntheses, Structures, Kinetics, and Catalysis [Abstract]

    A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu2 in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔHstorage. Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1′,3,3′‐tetra‐tert‐butyl (4), 1,2,2′,3′‐tetraphenyl (9), diiron (28), diosmium (24), mixed iron‐ruthenium (27), dimolybdenum (29), and ditungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3–SiO2 as a good candidate, although catalyst decomposition remains a challenge.

    K. Börjesson, D. Ćoso, V. Gray, J. C. Grossman, J. Guan, C. B. Harris, N. Hertkorn, Z. Hou, Y.Kanai, D. Lee, J. P. Lomont, A. Majumdar, S. K. Meier, K. Moth-Poulsen, R. L. Myrabo, S. C. Nguyen, R. A. Segalman, V. Srinivasan, W. B. Tolman, N. Vinokurov, K. P. C. Vollhardt, and T. W. Weidman
    Chem. Eur. J. 20, 15587 (2014)

2013

  1. Photoinduced Charge Transfer in Solvated Anthraquinones is Facilitated by Low-frequency Ring Deformations [Abstract]

    Efficiency of photoinduced charge transfer (CT) reactions in supramolecular assemblies is often compromised due to the rigid parametrization of acceptable donor–acceptor (D–A) distances and orientations. Using a combined approach of time-resolved optical measurements and electronic structure studies guided by molecular dynamics (MD) simulations, we demonstrate that dynamic control on CT rate exists for quenching of hydroxy-anthraquinones in donor aniline-like solvents. We find that anthraquinone ring deformation modes control the kinetics of CT within ∼300–700 fs by modulating the D–A electronic couplings. Our work demonstrates that low-frequency motions have to be critically understood for all such D–A pairs, and generates a rational methodology for utilization of hydroxy-anthraquinone acceptors in numerous photoactive architectures.

    A. Jha, , D. Chakraborty, V. Srinivasan and J. Dasgupta
    J. Phys. Chem. B 117, 12276 (2013)

2012

  1. Interplay between Intrinsic Defects, Doping, and Free Carrier Concentration in Perovskite Oxide Thin Films [Abstract]

    Using both computational and experimental analysis, we demonstrate a rich point-defect phase diagram in doped strontium titanate as a function of thermodynamic variables such as oxygen partial pressure and electronic chemical potential. Computational modeling of point-defect energetics demonstrates that a complex interplay exists between dopants, thermodynamic parameters, and intrinsic defects in thin films of SrTiO3 (STO). We synthesize STO thin films via pulsed laser deposition and explore this interplay between intrinsic defects, doping, compensation, and carrier concentration. Our point-defect analysis (i) demonstrates that careful control over growth conditions can result in the tunable presence of anion and cation vacancies, (ii) suggests that compensation mechanisms will pose intrinsic limits on the dopability of perovskites, and (iii) provides a guide for tailoring the properties of doped perovskite thin films.

    E. Ertekin, V. Srinivasan, J. Ravichandran, P. B. Rossen, W. Siemons, A. Majumdar, R. Ramesh, and J. C. Grossman
    Phys. Rev. B 85, 195460 (2012)

2011

  1. The Isotope-Effect in the Phase Transition of KH2PO4: New Insights from ab-initio Path-Integral Simulations [Abstract]

    We investigate the quantum-mechanical localization of 1H and 2H isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics simulations to obtain the nuclear real-space and momentum-space densities n(R) and n(k) of 1H and 2H, of which the latter densitites are compared to experimental neutron compton scattering data. Our results suggest a qualitative difference in the nature of the paraelectric phase in KDP between the two isotopes. Whereas both paraelectric states result from quantum delocalization, the essential difference is the change from a probably coherent to incoherent tunneling behavior of the hydrogen atoms across the hydrogen-bonds.

    V. Srinivasan , and D. Sebastiani
    J. Phys. Chem. C 115, 12631 (2011)
  2. Single-Molecule-Resolved Structural Changes Induced by Temperature and Light in Surface-Bound Organometallic Molecules Designed for Energy Storage [Abstract]

    We have used scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory calculations to investigate thermal and photoinduced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules (designed for light energy storage) on a Au(111) surface. We find that both the parent complex and the photoisomer exhibit striking thermally induced structural phase changes on Au(111), which we attribute to the loss of carbonyl ligands from the organometallic molecules. Density functional theory calculations support this conclusion. We observe that UV exposure leads to pronounced structural change only in the parent complex, indicative of a photoisomerization reaction.

    J. Cho, L. Berbil-Bautista, I. V. Pechenezhskiy, N. Levy, S. K. Meier, V. Srinivasan, Y. Kanai, J. C. Grossman, K. P. C. Vollhardt, and M. F. Crommie
    ACS Nano 5, 3701 (2011)

2010

  1. Mechanism of Thermal Reversal of the (Fulvalene)tetracabonyldiruthenium Photoisomerization: Toward Molecular Solar-Thermal Energy Storage
    Y. Kanai, V. Srinivasan, J. C. Grossman, S. K. Meier, and P. C. Vollhardt
    Angewandte Chemie International Edition 49, 8926 (2010)

Before 2010

  1. Strain engineering andone-dimensional organization of metal-insulator domains in single crystal VO2 beams [Abstract]

    Correlated electron materials can undergo a variety of phase transitions, including superconductivity, the metal–insulator transition and colossal magnetoresistance. Moreover, multiple physical phases or domains with dimensions of nanometres to micrometres can coexist in these materials at temperatures where a pure phase is expected. Making use of the properties of correlated electron materials in device applications will require the ability to control domain structures and phase transitions in these materials. Lattice strain has been shown to cause the coexistence of metallic and insulating phases in the Mott insulator VO2. Here, we show that we can nucleate and manipulate ordered arrays of metallic and insulating domains along single-crystal beams of VO2 by continuously tuning the strain over a wide range of values. The Mott transition between a low-temperature insulating phase and a high-temperature metallic phase usually occurs at 341 K in VO2, but the active control of strain allows us to reduce this transition temperature to room temperature. In addition to device applications,the ability to control the phase structure of VO2 with strain could lead to a deeper understanding of the correlated electron materials in general.

    J. Cao, E. Ertekin, V. Srinivasan, W. Fan, S. Huang, H. Zheng, J. W. L. Yim,D. R. Khanal, D. F. Ogletree, J. C. Grossman, and J. Wu
    Nature Nanotechnology 4, 732 (2009)
  2. Adsorption-induced surface stresses in alkanethiolate-Au self-assembled monolayers [Abstract]

    First-principles calculations were employed to elucidate the origin of adsorption-induced surface stresses in alkanethiolate self-assembled monolayers on an Au(111) surface. Our results suggest a mechanism that accounts for the huge relief of the tensile stress compared to the bare surface in terms of a local rearrangement of surface Au atoms accompanying charge removal from the surface towards the Au-S bond. A purely interadsorbate interaction model is shown to be inconsistent with the anisotropy and the magnitude of the calculated stress.

    V. Srinivasan, Giancarlo Cicero, and Jeffrey C. Grossman
    Phys. Rev. Lett. 101, 185504 (2008)
  3. The Proton Momentum Distribution in Water: An Open Path Integral Molecular Dynamics Study [Abstract]

    Recent neutron Compton scattering experiments have detected the proton momentum distribution in water. The theoretical calculation of this property can be carried out via “open” path integral expressions. In this work, present an extension of the staging path integral molecular dynamics method, which is then employed to calculate the proton momentum distributions of water in the solid, liquid, and supercritical phases. We utilize a flexible, single point charge empirical force field to model the system’s interactions. The calculated momentum distributions depict both agreement and discrepancies with experiment. The differences may be explained by the deviation of the force field from the true interactions. These distributions provide an abundance of information about the environment and interactions surrounding the proton.

    J. Morrone, V. Srinivasan, Daniel Sebastiani, and Roberto Car
    J.Chem. Phys. 126, 234504 (2007)
  4. PbTiO3 at Finite Temperature: An Ab‐initio Molecular Dynamics Study [Abstract]

    PbTiO3 is a prototypical ferroelectric material that exhibits a single structural phase transition (cubic to tetragonal): it is a soft mode driven, predominantly displacive, transition. In this paper, we study the behavior of PbTiO3 at finite temperature by ab‐initio molecular dynamics simulations. In this approach classical mechanics is used to describe nuclear dynamics, while the interatomic potential is generated on the fly from the ground state of the electrons within density functional theory. Fluctuations of volume and shape of the simulation cell are included by means of Parrinello‐Rahman constant pressure scheme. Extensive convergence studies based on static calculations indicate that a 3×3×3 supercell containing 135 atoms, with a single k‐point sampling, is sufficient to represent accurately the T = 0 energetics of this material. Although computationally demanding, ab‐initio molecular dynamics simulations for PbTiO3 using a 3×3×3 cell are feasible with current computational methodologies. Here we report preliminary results of simulations that are both below and above the phase‐transition temperature. We discuss, in particular, how phonon softening occurs with temperature and how thermal expansion affects the results.

    V. Srinivasan, R. Gebauer, R. Resta and R. Car
    AIP Conference Proceedings 677, 168 (2003)