New dipole moment surfaces of methane Andrei V. Nikitin a,b,⇑ , Michael Rey b , Vladimir G. Tyuterev b a Laboratory of Theoretical Spectroscopy, V.E. Zuev Institute of Atmospheric Optics, SB RAS, 1 Academician Zuev Square, 634021 Tomsk, Russia b Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR CNRS 6089, Université de Reims, U.F.R. Sciences, B.P. 1039, 51687 Reims Cedex 2, France article info Article history: Received 12 December 2012 In final form 12 February 2013 Available online 26 February 2013 abstract New dipole moment surfaces (DMS) of methane are constructed using extended ab initio CCSD(T) calcu- lations at 19 882 nuclear configurations. The DMS analytical representation is determined through an expansion in symmetry adapted products of internal nonlinear coordinates involving 967 parameters up to the 6th order. Integrated intensities of seven lower polyads up to J = 30 for 12 CH 4 and 13 CH 4 are in a good agreement with the HITRAN 2008 database, and with other available experimental data. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Methane is a high symmetry hydrocarbon, which is important in numerous fields of science. Acting as a greenhouse gas of the earth atmosphere, CH 4 is also a significant constituent of various planetary atmospheres, like those of the Giant Planets [1,2] (Jupi- ter, Saturn, Uranus and Neptune) and of Titan [3,4] (Saturn’s main satellite). The knowledge of the CH 4 opacity is also very important for the modeling of brown dwarfs and for other astrophysical applications. As the remote sensing via infrared spectroscopy is generally the best diagnostic tool to study CH 4 in these environ- ments, it appears essential to be able to model its absorption very precisely. The analysis of highly excited vibration–rotation energy levels and transitions of the methane molecule is a difficult prob- lem due to complex structures of vibrational polyads, numerous resonance couplings and high dimensionality of the calculation models [5–8]. Knowledge of the molecular dipole moment surface (DMS) would help resolving many of related issues. High temperature measurements [9,10] of methane spectra were not yet fully ana- lyzed as this requires much more accurate theoretical predictions of ro-vibrational spectra. A certain progress in the dipole moment calculation from ab initio theory has been achieved [11–14]. Signo- rell et al. [15] have determined six parameters in an approximation of the methane DMS containing four valence depending and two angle depending terms. This DMS form has been applied by Marqu- ardt and Quack [13] for the calculation of some integrated cross sec- tions of deuterated methane isotopologues. Warmbier et al. [16] calculated an ab initio methane DMS and methane spectra at T = 1000 K using MULTIMODE program [17]. Recently Cassam-Chenai and Lievin [18] computed a third order dipole moment normal mode expansion and calculated line intensities for rotational tran- sitions of methane within the vibrational ground state. However a quantitatively accurate modeling of intensities of excited ro- vibrational states of methane still remains a difficult problem to be solved. Several issues specific to the methane molecule have to be addressed in the context of current spectroscopic applications: (a) The first challenge is to extend accurate ab initio calculations to sufficiently dense grids of nuclear configurations and to obtain a precise fit of ab initio points with an appropriate reliable analytical DMS representation accounting for molecular symmetry properties; (b) Another one is to assure the convergence of calculations of ro-vibrational levels and eigenfunctions from molecular potential energy surface (PES) as well as of transition moments using quantum mechanical, variationally-based approaches up to higher energy ranges. This Letter is a part of a long term effort to extend spectroscopic data analyses and calculations for isotopologues of the methane molecule in the infrared range [19–21]. The aim of this Letter is an accurate calculation of the methane DMS in internal coordinates for a large extent of nuclear configurations. In order to check the validity of our DMS the integrated intensities of seven lower poly- ads were calculated, and compared with experimental databases. 2. Electronic structure calculations and determination of ab initio DMS In order to obtain accurate calculations of intensities of vibra- tion–rotation transitions from theoretical DMS surfaces which would be useful for spectroscopic analyses, it is necessary to com- bine high level ab initio methods with sufficiently large basis sets in electronic structure calculations. The coupled cluster approach including single and double excitations and the perturbative 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.022 ⇑ Corresponding author at: Laboratory of Theoretical Spectroscopy, V.E. Zuev Institute of Atmospheric Optics, SB RAS, 1 Academician Zuev Square, 634021 Tomsk, Russia. E-mail address: avn@lts.iao.ru (A.V. Nikitin). Chemical Physics Letters 565 (2013) 5–11 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett