A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field

A systematic method to fit calculated to observed vibrational frequencies h as been developed and implemented in a computer program. The procedure cons ists of the refinement of a scaled quantum mechanical force field (SQMFF) p reviously obtained according to Pulay's method. The key step in the process is the generation of an intermediate matrix, C LambdaC(T), which is then r efined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. T his scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force f ields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests. (C) 2000 American Institute of Physics. [S00 21-9606(00)31338-1].