A SPECTROSCOPICALLY DETERMINED POTENTIAL-ENERGY SURFACE FOR THE GROUND-STATE OF (H2O)-O-16 - A NEW LEVEL OF ACCURACY.
J CHEM PHYS
7651 - 7657.
The potential energy function for the electronic ground state of the water molecule has been obtained by fitting rotation-vibration term values involving J less than or equal to 14 for 24 vibrational states of (H2O)-O-16 together with 25 additional vibrational term values belonging to higher excited states. The fitting was carried out by means of an exact kinetic energy Hamiltonian. It was found that the differences between the exact kinetic energy calculations and calculations with the MORBID program (i.e., calculations with an approximate kinetic energy operator) depend only very slightly on the parameters of the potential. This fact allowed us to make an inexpensive fitting using the MORBID approach and still get the accuracy obtainable with the exact kinetic energy Hamiltonian. The standard deviation for 1600 term values was 0.36 cm(-1). For 220 ground state energy levels the standard deviation was 0.03 cm(-1). With the fitted potential, calculations of term values with J less than or equal to 35 were carried out. This showed the excellent predictive power of the new potential. For the J=20 term values in the vibrational ground state, the deviations from experiment are typically below 0.2 cm(-1).The discrepancy for the observed level with the highest K-a value, J(KaKc) = 20(200), is only 0.008 cm(-1). The calculated term value for the observed level with the highest J, 35(035), deviates 0.1 cm(-1) cm from experiment. Because of the level of accuracy achieved in these calculations, we can for the first time demonstrate the breakdown of the Born-Oppenheimer approximation for the water molecule. The high K-a level calculations allow us to show that the rotational energy level structure in water is at least of a very different nature than the fourfold cluster structures observed for H2Se and calculated for H2S, H2Se, and H2Te.
|Title:||A SPECTROSCOPICALLY DETERMINED POTENTIAL-ENERGY SURFACE FOR THE GROUND-STATE OF (H2O)-O-16 - A NEW LEVEL OF ACCURACY|
|Keywords:||INTERACTING VIBRATIONAL-STATES, BORN-OPPENHEIMER APPROXIMATION, SUBMILLIMETER-WAVE SPECTRUM, HYDROGEN FLAME SPECTRA, LINE POSITIONS, TRANSITION FREQUENCIES, TRIATOMIC-MOLECULES, WATER MOLECULE, H2O, STRENGTHS|
|UCL classification:||UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Maths and Physical Sciences
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