A new ONIOM implementation in Gaussian98. Part I. The calculation of energies, gradients, vibrational frequencies and electric field derivatives

The IMOMM, IMOMO, and ONIOM methods have been proven to be powerful tools f or the theoretical treatment of large molecular systems where different lev els of theory are applied to different parts of a molecule. Within this fra mework we present a modified handling of the link atoms which are introduce d to terminate the dangling bonds of the model system. Using this new schem e the definition of the combined energy gradient, the Hessian matrix, and t he integration of higher derivatives of the energy with respect to nuclear coordinates and the electric field Vector becomes straightforward. This all ows for the first time the consistent combination of vibrational frequencie s and the calculation of other molecular properties such as IR intensities, Raman intensities as well as dipole moments, polarizabilities, and hyperpo larizabilities. Test calculations for some typical as well as unusual examp les and partitioning schemes are presented to demonstrate the power and lim itations of the method and to provide guidelines for its applicability. Use rs of the method are strongly advised to test, calibrate and confirm for th emselves the validity of the method combination and the model subsystem for the properties they want to calculate. (C) 1999 Elsevier Science B.V. All rights reserved.