46 research outputs found
Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo
We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene
using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We
consider the , , and structures as the best candidates
for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC)
calculations to obtain accurate static phase diagrams as benchmarks for modern
van der Waals density functionals. We use density functional perturbation
theory to compute phonon contribution in the free-energy calculations. Our DFT
enthalpy-pressure phase diagram indicates that the and
structures are the most stable phases within the studied pressure range. The
DMC Gibbs free-energy calculations predict that the room temperature to
phase transition occurs at 2.1(1) GPa. This prediction is consistent
with available experimental results at room temperature. Our DMC calculations
show an estimate of 50.60.5 kJ/mol for crystalline benzene lattice energy
Chemical accuracy from quantum Monte Carlo for the Benzene Dimer
We report an accurate study of interactions between Benzene molecules using
variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC)
methods. We compare these results with density functional theory (DFT) using
different van der Waals (vdW) functionals. In our QMC calculations, we use
accurate correlated trial wave functions including three-body Jastrow factors,
and backflow transformations. We consider two benzene molecules in the parallel
displaced (PD) geometry, and find that by highly optimizing the wave function
and introducing more dynamical correlation into the wave function, we compute
the weak chemical binding energy between aromatic rings accurately. We find
optimal VMC and DMC binding energies of -2.3(4) and -2.7(3) kcal/mol,
respectively. The best estimate of the CCSD(T)/CBS limit is -2.65(2) kcal/mol
[E. Miliordos et al, J. Phys. Chem. A 118, 7568 (2014)]. Our results indicate
that QMC methods give chemical accuracy for weakly bound van der Waals
molecular interactions, comparable to results from the best quantum chemistry
methods.Comment: Accepted for publication in the Journal of Chemical Physics, Vol.
143, Issue 11, 201
Resonating Valence Bond Quantum Monte Carlo: Application to the ozone molecule
We study the potential energy surface of the ozone molecule by means of
Quantum Monte Carlo simulations based on the resonating valence bond concept.
The trial wave function consists of an antisymmetrized geminal power arranged
in a single-determinant that is multiplied by a Jastrow correlation factor.
Whereas the determinantal part incorporates static correlation effects, the
augmented real-space correlation factor accounts for the dynamics electron
correlation. The accuracy of this approach is demonstrated by computing the
potential energy surface for the ozone molecule in three vibrational states:
symmetric, asymmetric and scissoring. We find that the employed wave function
provides a detailed description of rather strongly-correlated multi-reference
systems, which is in quantitative agreement with experiment.Comment: 5 page, 3 figure
Systematic study of finite-size effects in quantum Monte Carlo calculations of real metallic systems
A consistent description of the iron dimer spectrum with a correlated single-determinant wave function
We study the iron dimer by using an accurate ansatz for quantum chemical
calculations based on a simple variational wave function, defined by a single
geminal expanded in molecular orbitals and combined with a real space
correlation factor. By means of this approach we predict that, contrary to
previous expectations, the neutral ground state is while the ground
state of the anion is , hence explaining in a simple way a long
standing controversy in the interpretation of the experiments. Moreover, we
characterize consistently the states seen in the photoemission spectroscopy by
Leopold \emph{et al.}. It is shown that the non-dynamical correlations included
in the geminal expansion are relevant to correctly reproduce the energy
ordering of the low-lying spin states.Comment: 5 pages, submitted to the Chemical Physics Letter
Electronic structures and thermal properties of 312-MAX phases
The Mn+1AXn phases (n=1,2, or 3) or MAX phases , where M is a transition metal, A is an A- group element, and X is either C or N or both, exhibit particular chemical, physical, electrical, and mechanical properties. The unusual properties of the MAX phases can be linked to their layered structures and the nature of bonding. The M-X bonds are strong, while M-A bonds are relatively weak. These mixed metallic-covalent bondings are the source of many exceptional properties of the
MAX phases.
In this work we study a new discovered MAX phase of Zr3AlC2, which according to general formula of Mn+1AXn, it belongs to the 312 stoichiometry group. We employ Density Functional Theory (DFT)-based methods to obtain electronic structure and lattice dynamics properties. The quasi-harmonic approximation is used to calculate the Helmholtz free energy at temperature range from 10 \u3c T \u3c 1200 K. For the first time, we predict coefficient of thermal expansion for Zr3AlC2 MAX phase. We discuss details and technicalities which are required for accurate calculations of lattice vibration contribution to thermodynamic free energy
Unconventional phase III of high-pressure solid hydrogen
We reassess the phase diagram of high-pressure solid hydrogen using
mean-field and many-body wave function based approaches to determine the nature
of phase III of solid hydrogen. To discover the best candidates for phase III,
density functional theory calculations within the meta-generalized gradient
approximation by means of the strongly constrained and appropriately normed
(SCAN) semilocal density functional are employed. We study eleven molecular
structures with different symmetries, which are the most competitive phases,
within the pressure range of 100 to 500~GPa. The SCAN phase diagram predicts
that the and structures are the best candidates for phase
III with an energy difference of less than 1~meV/atom. To verify the stability
of the competitive insulator structures of and , we apply
the diffusion Monte Carlo (DMC) method to optimise the percentage of
exact-exchange in the trial many-body wave function. We found that the
optimised equals to , and denote the corresponding exchange and
correlation functional as PBE1. The energy gain with respect to the well-known
hybrid functional PBE0, where , varies with density and
structure. The PBE1-DMC enthalpy-pressure phase diagram predicts that the
structure is stable up to 210~GPa, where it transforms to the
. Hence, we predict that the phase III of high-pressure solid hydrogen
is polymorphic.Comment: Accepted for publication in Phys. Rev.
Correlation energy of the spin-polarized electron liquid by quantum Monte Carlo
Variational and diffusion quantum Monte Carlo (VMC and DMC) methods with
Slater-Jastrow-backflow trial wave functions are used to study the
spin-polarized three-dimensional uniform electron fluid. We report ground state
VMC and DMC energies in the density range .
Finite-size errors are corrected using canonical-ensemble twist-averaged
boundary conditions and extrapolation of the twist-averaged energy per particle
calculated at three system sizes (N=113, 259, and 387) to the thermodynamic
limit of infinite system size. The DMC energies in the thermodynamic limit are
used to parameterize a local spin density approximation correlation function
for inhomogeneous electron systems.Comment: arXiv admin note: substantial text overlap with arXiv:2209.1022
Dissociation of high-pressure solid molecular hydrogen: Quantum Monte Carlo and anharmonic vibrational study
A theoretical study is reported of the molecular-to-atomic transition in
solid hydrogen at high pressure. We use the diffusion quantum Monte Carlo
method to calculate the static lattice energies of the competing phases and a
density-functional-theory-based vibrational self-consistent field method to
calculate anharmonic vibrational properties. We find a small but significant
contribution to the vibrational energy from anharmonicity. A transition from
the molecular Cmca-12 direct to the atomic I4_1/amd phase is found at 374 GPa.
The vibrational contribution lowers the transition pressure by 91 GPa. The
dissociation pressure is not very sensitive to the isotopic composition. Our
results suggest that quantum melting occurs at finite temperature.Comment: Accepted for publication by Phys. Rev. Let