Ab initio effective rotational Hamiltonians: A comparative study

Abstract

Two independent methods to obtain ab initio effective rotational Hamiltonians have been implemented recently. The first one is based on a generalization of perturbation theory to noncommutative rings, the other one on contact transformation (CT) techniques. In principle, both methods are able to give rotational Hamiltonians including centrifugal distortion effects of arbitrary high orders. These methods are compared, for the first time, in this article with regard to calculations of the rotational levels of methane vibrational ground state.

First theoretical global line lists of ethylene (12C2H4) spectra for the temperature range 50−700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres

“…Ab initio studies of small polyatomic molecules emerged in the past decades with drastically improved computational methods and hardware resources. A breakthrough in predicting rotationally resolved spectra of small molecules was made possible by accurate electronic structure calculations of the molecular PESs and DMSs (see, e.g., Partridge & Schwenke 1997;Cours et al 2002;Yurchenko et al 2009;Nikitin et al 2009;Lodi et al 2011;Császár et al 2010;Szalay et al 2011;Huang et al 2011aHuang et al , 2012Sousa-Silva et al 2014;Nikitin et al 2014;Li et al 2014;Delahaye et al 2015) and by developing efficient methods for quantum nuclear motion calculations (see Schwenke & Partridge 2001;Schwenke 2002;Cassam-Chenaï et al 2012;Yurchenko et al 2007;Wang & Carrington Jr. 2013a,b;Carter et al 2009;Rey et al 2012;Bowman et al 2005). In case of triatomics, such as water isotopic species (Yurchenko et al 2009;Barber et al 2011), carbon dioxide (Huang et al 2013(Huang et al , 2014, and ozone accurate theoretical predictions led to significant extensions of spectral analyses toward higher energy ranges.…”

An improved third order dipole moment surface for methane

“…The ro-vibrational Hamiltonian has been diagonalized by means of a combination of the VMFCI method with a generalized perturbation theory [28,35,44]. In fact, this approach can be seen as a particular case of a unified method, called the generalized mean field configuration interaction (GMFCI).…”

“…Note that some of these PES have been partly refined using experimental data. In the present study, we have used the NRT PES transformed to mass-weighted Cartesian normal coordinates up to order 10, already employed in our previous studies [28,29].…”

“…The VMFCI method has been combined with a generalized perturbation theory [28,35,44], to obtain effective rotational observable, from which accurate rotational spectra can be calculated [29,45].…”

From atoms to biomolecules: a fruitful perspective

“…Extensive calculations were also performed on methane [25][26][27][28][29], with the goal to predict the rotation-vibration spectrum which is of particular interest for evaluating methane abundance in planetary atmospheres. Our most recent results use an accurate potential energy surface calculated at the CCSD(T)/ACV5Z level of theory [119] and a dipole moment surface that we calculated at the IC-MRCI/ ACV5Z level.…”