| In modern theoretical chemistry, the importance of evaluating force fields from reliable wavefunctions has become increasingly apparent. The second derivatives matrix (Hessian) of the total energy of a system with respect to geometrical variables is used not only to evaluate the vibrational frequencies and related properties, but also to develop the quantum force field. The third derivatives of electronic energy with respect to electric field components can determine the nonlinear optical properties.; We first systematically exploit the pseudospectral (PS) theory along this direction. The driver to compute the molecular nonlinear optical properties (α and β) with the PS method at Hartree-Fock (HF) level has been completed. The CPU time compared to the conventional driver (e.g. Gaussian 92) scales much better with the system size. The code to compute molecular vibrational frequencies implemented with the PS method and conventional analytical methods in the frameworks of closed-shell Hartree-Fock (HF), general restricted open-shell Hartree-Fock, as well as unrestricted open-shell Hartree-Fock has been finished. The computation shows that PS theory can be significantly more successful than the conventional quantum chemistry (CQC) method.; Density functional theory (DFT) has emerged in recent years as a promising alternative to conventional ab initio methods in quantum chemistry, not only because it scales as N3 (N is the number of basis functions of the system), but also because both the exchange and the correlation energy are included. We firstly combine the PS theory with DFT theory, and apply this hybrid method to generate the molecular hessian. The results from this method are much better than the results from the Hartree-Fock method. |
