The molecular potential energy surface and vibrational energy levels of methyl fluoride. Part II

Abstract

New analytical bending and stretching, ground electronic state, potential energy surfaces for CH(3)F are reported. The surfaces are expressed in bond-length, bond-angle internal coordinates. The four-dimensional stretching surface is an accurate, least squares fit to over 2000 symmetrically unique ab initio points calculated at the CCSD(T) level. Similarly, the five-dimensional bending surface is a fit to over 1200 symmetrically unique ab initio points. This is an important first stage towards a full nine-dimensional potential energy surface for the prototype CH(3)F molecule. Using these surfaces, highly excited stretching and (separately) bending vibrational energy levels of CH(3)F are calculated variationally using a finite basis representation method. The method uses the exact vibrational kinetic energy operator derived for XY(3)Z systems by Manson and Law (preceding paper, Part I, Phys. Chem. Chem. Phys., 2006, 8, DOI: 10.1039/b603106d). We use the full C(3v) symmetry and the computer codes are designed to use an arbitrary potential energy function. Ultimately, these results will be used to design a compact basis for fully coupled stretch-bend calculations of the vibrational energy levels of the CH(3)F system.

Calculation of vibrational energy levels of symmetric molecules from potential energy surface

“…Äëÿ íåêîòîðûõ òðåõàòîìíûõ ìîëåêóë [1,[13][14][15][16], íàïðèìåð âîäû, òî÷íîñòü ðàñ÷åòîâ, îñíîâàííûõ íà êâàíòîâîé õèìèè è âàðèàöèîííûõ ìåòîäàõ, ïîçâîëÿåò èäåíòèôèöèðîâàòü ñëîaeíûå ýêñïåðèìåíòàëüíûå ñïåêòðû. Çàìåòíûé ïðîãðåññ â ïîñëåäíåå âðåìÿ äîñòèãíóò äëÿ ñèììåòðè÷íûõ ìíîãîàòîìíûõ ìîëåêóë, íàïðèìåð ôîñôèíà [4,[17][18][19][20], àììèàêà [7,[21][22][23], ïåðåêèñè âîäîðîäà [24,25], ôîðìàëüäåãèäà [26], àöåòèëåíà [27,28], ìåòàíà [2,5,6,[29][30][31][32][33][34], ôòîðìåòàíà [35,36], õëîðìåòàíà [37], ýòèëåíà [38][39][40], CH 3 OH [41], CH 5 + [42], à òàêaeå äðóãèõ ìîëåêóë [43][44][45] (ñì. òàêaeå ññûëêè â íèõ).…”

A general rigorous quantum dynamics algorithm to calculate vibrational energy levels of pentaatomic molecules

“…The third set of coordinates (Icd = 3) are formed by the combined (3 + 1) Radau and Jacobi vectors, in which the Cartesian vectors of atoms can be expressed as [49] x A ¼ r 1 þPA;…”

“…In particular, both new Lanczos algorithms utilized the merits of quantum mechanical Hamiltonian in a set of orthogonal scattering coordinates. That is, the resulting Hamiltonian is very compact with a partitioned structure, and has a common formulation except for the mass-dependent prefactors regardless of types of orthogonal vectors [11,13,25,34,36,49,[66][67][68]. Otherwise, if non-orthogonal coordinates [25, [69][70][71][72][73][74][75][76] are used, the kinetic energy operators obtained are rather complicated.…”

“…Both WAVR4 and TetraVib programs are able to study the large amplitude motions of molecules. For larger polyatomic systems, a few powerful programs have also been developed with minor dynamics approximations, based on the multiconfiguration time-dependent Hartree (MCTDH) method [38][39][40][41][42][43], vibrational self-consistent field/configuration interaction (VSCF/CI) method [44][45][46][47][48] and reduced-dimensionality approach [19,49,50]. These programs are applicable for molecules with up to about 20 atoms.…”

The rotation–vibration spectrum of methyl fluoride from first principles

“…20,21 Although the rovibrational spectrum of CH 3 F has been well documented, its theoretical description is not reflective of the current state-of-the-art in variational calculations. Notable recent works include the PESs and energy level computations of Manson et al 22,23 , Nikitin, Rey, and Tyuterev 24 and Zhao et al 25,26 . Theoretical CH 3 F spectra are also available from the TheoReTS database 27 for a temperature range of 70-300 K but details on the calculations are unpublished except for the PES.…”