arXiv subjects

Anoop Ajaya Kumar Nair

Publications and source records attributed to Anoop Ajaya Kumar Nair.

3 recordsLinked to original sources

arXiv2026

DFT GGA based datasets for H$_2$O potential energy surfaces, permanent moment and polarizability tensors

We present in this article data on the computed potential energy surface, permanent moment surface (from dipole to hexadecapole) and polarizability surface (from dipole-dipole to quadrupole-quadrupole) for the H$_2$O molecule, calculated at the level of the generalized gradient approximation (GGA) in Kohn-Sham density functional theory (KS-DFT) in the grid-based projector augmented wave code GPAW, using the charge perturbation code C-pol. The GGA-based functionals are PBE, RPBE and BLYP. The monomer surfaces are composed of systematic, symmetry-reduced deformations of the H$_2$O monomer and comprise 10,000 configurations per functional, with traceless Cartesian tensor moment values reported as data strings in the xyz file format. In addition, we provide cluster datasets for n-H$_2$O aggregates (n = 2-6), sampled from liquid-like configurations and computed with the same KS-DFT settings, where each extended xyz file encodes the total interaction energy of the cluster in the comment line. Finally, ab initio molecular dynamics (AIMD) trajectories of periodic liquid water boxes, containing between 25 to 150 H$_2$O molecules at ambient conditions, are included, furnishing time series of atomic configurations and energies that are consistent with the monomer and cluster data and suitable for validating classical and machine-learned water models.

physics.chem-ph↗

arXiv2026

C-Pol: Point charge perturbation scheme for mapping tensor moment surfaces

We present an efficient moment-based perturbation scheme for evaluating polarizability tensors of small molecules at a fraction of the computational cost of conventional energy-based approaches. Rather than applying explicit electric fields, the method perturbs the molecular charge density using strategically arranged external point charges, as in QM/MM simulations, and extracts polarizability tensors from finite differences of multipole moments extrapolated to the zero-perturbation limit. C-Pol is implemented as a backend-agnostic Python package requiring only that the host quantum chemistry code support external point-charge potentials. We demonstrate interfaces with three codes spanning complementary numerical approaches: GPAW (real-space grids), NWChem, and PySCF (atom-centered basis sets with complete basis set extrapolation). Validation against Gaussian 16 energy-based reference calculations across molecules representing all 19 commonly occurring point groups yields agreement within 3%, with the largest deviations confined to tensor components that are numerically small and contribute negligibly to the electrostatic potential. Finite-difference polarizabilities computed via C-Pol are further shown to agree with analytic coupled-perturbed Hartree-Fock values to within 0.02 a.u. across all tested basis sets, at comparable computational cost for medium-sized systems and with more favorable scaling for larger ones. The package provides a practical and transferable route to high-quality multipole polarizability data for force field development, polarizable embedding, and machine-learning training sets.

physics.chem-ph↗

arXiv2026

Polarizable Embedding QM/MM for Periodic Systems

A general polarizable embedded (PE) quantum mechanics/molecular mechanics scheme for periodic systems is presented, describing mutual polarization of the two subsystems. The QM system, described with density functional theory (DFT), is coupled to a single center multipole expansion (SCME) model, characterising H$_2$O molecules in the MM region. In SCME the H$_2$O molecules are ascribed anisotropic dipole and quadrupole polarizabilities and permanent multipoles up to and including the hexadecapole. Our embedding scheme illustrates a smooth and efficient convergence pattern of the periodic interaction potential by introducing a single and clustered multipole expansion points in the far-field. By choosing the near- and far-field expansion of the potential carefully the PE-QM/MM calculation matches the level of accuracy of a the QM calculation. In the short range, the electrostatic interaction between the QM and MM subsystems is damped with a real-space and pair-wise isotropic damping functions - resulting in a screened interaction and preventing over-polarization. In molecular dynamics simulations the two subsystems are separated with the elastic scattering assisted flexible inner region [Kirchhoff et. al. JCTC, 2021, 17, 9, 5863] - ensuring a smooth transition in the radial distribution at the boundary between the two subsystems.

physics.chem-ph↗