arXiv subjects

Vishikh Athavale

Publications and source records attributed to Vishikh Athavale.

5 recordsLinked to original sources

arXiv2026

Shadow Molecular Dynamics for General Nonlinear Flexible-Charge Models

Machine-learned interatomic potentials have greatly expanded the reach of molecular dynamics simulations, but complex chemical environments often require explicit treatment of long-range electronic response, including charge transfer and polarization. Physics-based flexible-charge models capture these effects but require global nonlinear charge equilibration at every time step to obtain Born-Oppenheimer energies and forces. This process is computationally expensive and can lead to instabilities unless tightly converged. Here, we introduce a shadow molecular dynamics framework for flexible-charge models with arbitrary local nonlinearities and long-range multipolar electrostatics. We replace the exact Born-Oppenheimer potential with a nearby shadow potential obtained by exactly equilibrating a partially linearized electronic energy functional. This provides conservative forces and reduces the global nonlinear equilibration to a quasi-diagonal linear problem that can be solved directly at low cost. Auxiliary dynamical electronic degrees of freedom, propagated through an extended-Lagrangian dynamics, capture the long-range electronic response and define the reference state for the linearization of the energy functional. GPU-accelerated simulations with over 100,000 atoms demonstrate that the shadow potential closely follows the tightly converged Born-Oppenheimer surface, while maintaining long-term stability at a computational cost approaching that of a fixed-charge model. These advances establish an efficient and broadly applicable framework for machine-learned interatomic potentials that explicitly incorporate long-range electrostatic interactions and environment-dependent electronic response.

physics.chem-ph↗

arXiv2025

PySEQM 2.0: Accelerated Semiempirical Excited State Calculations on Graphical Processing Units

We report the implementation of electronic excited states for semi-empirical quantum chemical methods at the configuration interaction singles (CIS) and time-dependent Hartree-Fock (TDHF) level of theory in the PySEQM software. Built on PyTorch, this implementation leverages GPU acceleration to significantly speed up molecular property calculations. Benchmark tests demonstrate that our approach can compute excited states for molecules with nearly a thousand atoms in under a minute. Additionally, the implementation also includes a machine learning interface to enable parameters re-optimization and neural network training for future machine learning applications for excited state dynamics.

physics.chem-ph↗

arXiv2023

Surface Hopping, Electron Translation Factors, Electron Rotation Factors, Momentum Conservation, and Size Consistency

For a system without spin-orbit coupling, the (i) nuclear plus electronic linear momentum and (ii) nuclear plus orbital electronic angular momentum are good quantum numbers. Thus, when a molecular system undergoes a nonadiabatic transition, there should be no change in the total linear or angular momentum. Now, the standard surface hopping algorithm ignores the electronic momentum and indirectly equates the momentum of the nuclear degrees of freedom to the total momentum. However, even with this simplification, the algorithm still does not conserve either the nuclear linear or the nuclear angular momenta. Here, we show that one way to address these failures is to dress the derivative couplings (i.e. the hopping directions) in two ways: (i) we disallow changes in the nuclear linear momentum by working in a translating basis (which is well known and leads to electron translation factors [ETFs]); and (ii) we disallow changes in the nuclear angular momentum by working in a basis that rotates around the center of mass (which is not well-known and leads to a novel, rotationally removable component of the derivative coupling that we will call electron rotation factors [ERFs] below, cf. Eq. 96). The present findings should be helpful in the short term as far as interpreting surface hopping calculations for singlet systems (without spin) and then developing new surface hopping algorithm in the long term for systems where one cannot ignore the electronic orbital and/or spin angular momentum.

physics.chem-ph↗

arXiv2022

On The Inclusion of One Double Within CIS and TD-DFT

We present an improved approach for generating a set of optimized frontier orbitals (HOMO and LUMO) that minimizes the energy of one double configuration. We further benchmark the effect of including such a double within a CIS or TD-DFT configuration interaction Hamiltonian for a set of test cases. We find that, although we cannot achieve quantitative accuracy, the algorithm is quite robust and routinely delivers an enormous qualitative improvement to standard single-reference electronic structure calculations.

physics.chem-ph↗

arXiv2019

Anharmonic Effects in the Low-Frequency Vibrational Modes of Aspirin and Paracetamol Crystals

The low-frequency range of vibrational spectra is sensitive to collective vibrations of the lattice. In molecular crystals, it can be decisive to identify the structure of different polymorphs, and in addition, it plays an important role on the magnitude of the temperature-dependent component of vibrational free energy differences between these crystals. In this work we study the vibrational Raman spectra and vibrational density of states of different polymorphs of the flexible Aspirin and Paracetamol crystals based on dispersion-corrected density-functional theory, density-functional perturbation theory, and $ab\,initio$ molecular dynamics. We examine the effect of quasi-harmonic lattice expansion and compare the results of harmonic theory and the time correlation formalism for vibrational spectra. Lattice expansion strongly affects the collective vibrations below 300 cm$^{-1}$, but it is significantly less important at higher frequencies, while thermal nuclear motion can be important in the full vibrational range. We also observe that the inclusion or neglect of many-body van der Waals dispersion interactions do not cause large differences in the low-frequency range of Raman spectra or vibrational density of states, provided the lattice constants are fixed. We obtain quantitative agreement with experimental room-temperature Raman spectra below 300 cm$^{-1}$ for all polymorphs studied. Examining the two-dimensional correlations between different vibrations, we find which modes show a larger degree of anharmonic coupling to others, providing a possible route to assess the accuracy of harmonic free energy evaluations in different cases.

physics.chem-ph↗