Ab initio potential energy surfaces for NH-NH with analytical long range

TL;DR

This paper develops detailed ab initio potential energy surfaces for the NH-NH complex across three spin states, incorporating long-range expansions and corrections, to improve understanding of molecular interactions.

Contribution

It provides new four-dimensional potential energy surfaces for NH-NH, including long-range behavior, based on high-level ab initio calculations and fitting techniques.

Findings

Good agreement of long-range coefficients with perturbation theory

Potential surfaces accurately describe interactions across spin states

Size-consistency correction improves potential accuracy

Abstract

We present four-dimensional ab initio potential energy surfaces for the three spin states of the NH-NH complex. The potentials are partially based on the work of Dhont et al. [J. Chem. Phys. 123, 184302 (2005)]. The surface for the quintet state is obtained at the RCCSD(T)/aug-cc-pVTZ level of theory and the energy diferences with the singlet and triplet states are calculated at the CASPTn/aug-cc-pVTZ (n = 2; 3) level of theory. The ab initio potentials are fitted to coupled spherical harmonics in the angular coordinates, and the long range is further expanded as a power series in 1/R. The RCCSD(T) potential is corrected for a size-consistency error prior to fitting. The long-range coeficients obtained from the fit are found to be in good agreement with perturbation theory calculations.