# A complete OSV-MP2 analytical gradient theory for molecular structure and dynamics simulations

## TL;DR

This paper introduces an exact algorithm for computing analytical gradients in OSV-MP2 theory, enabling accurate molecular structure and dynamics simulations with applications demonstrated on water cations and ethanol rotors.

## Contribution

It presents the first exact gradient algorithm for OSV-MP2 with explicit OSV relaxation, improving accuracy and enabling molecular dynamics simulations.

## Key findings

- High accuracy of gradients demonstrated on selected molecules
- Successful BOMD simulations of water cations using OSV-MP2
- Computed free energy surface of ethanol rotors

## Abstract

We propose an exact algorithm for computing the analytical gradient within the framework of the orbital-specific-virtual (OSV) second-order M{\o}ller-Plesset (MP2) theory in resolution-of-identity (RI) approximation. We implement the exact relaxation of perturbed OSVs through the explicit constraints of the perturbed orthonormality, the perturbed diagonality and the perturbed singular value condition. We explicitly show that the rotation of OSVs within the retained OSV subspace makes no contribution to gradients, as long as the iterative solution of the unperturbed Hylleraas residual equation is well converged. The OSV relaxation is solved as the perturbed non-degenerate singular value problem between the retained and discarded OSV subspaces. The detailed derivation and preliminary implementations for gradient working equations are discussed. The coupled-perturbed localization method is implemented for meta-L\"owdin localization function. The numerical accuracy of computed OSV-MP2 gradients is demonstrated for the geometries of selected molecules that are often discussed in other theories. Moreover, the OSV-MP2 analytical gradients can generate atomic forces that are utilized to drive the Born-Oppenheimer molecular dynamics (BOMD) simulation for studying structural and vibrational properties with respect to OSV selections. By performing the OSV-MP2 NVE BOMD calculation using the normal OSV selection, the structural and vibrational details of protonated water cations are well reproduced. The 200 picoseconds NVT well-tempered metadynamics at 300 K has been simulated to compute the OSV-MP2 rotational free energy surface of coupled hydroxyl and methyl rotors for ethanol molecule.