Vibrational transition moments of CH$_4$ from first principles

TL;DR

This paper introduces a new high-accuracy ab initio dipole moment surface for methane, enabling precise calculations of vibrational transition moments and IR band intensities, crucial for spectroscopic applications.

Contribution

It presents a novel 9D ab initio dipole moment surface for methane using coupled cluster methods and a symmetrized bond representation, combined with variational vibrational calculations.

Findings

Computed vibrational transition moments and IR intensities for methane.

Developed a new spectroscopic potential energy surface.

Enabled future generation of hot line lists for methane.

Abstract

A new nine-dimensional (9D), \textit{ab initio} electric dipole moment surface (DMS) of methane in its ground electronic state is presented. The DMS is computed using an explicitly correlated coupled cluster CCSD(T)-F12 method in conjunction with an F12-optimized correlation consistent basis set of the TZ-family. A symmetrized molecular bond representation is used to parameterise the 9D DMS in terms of sixth-order polynomials. Vibrational transition moments as well as band intensities for a large number of IR-active vibrational bands of $^{12}$CH$_4$ are computed by vibrationally averaging the \textit{ab initio} dipole moment components. The vibrational wavefunctions required for these averages are computed variationally using the program TROVE and a new `spectroscopic' $^{12}$CH$_4$ potential energy surface. The new DMS will be used to produce a hot line list for $^{12}$CH$_4$.