Calculation of electric field gradients in some linear molecules using semi-empirical scfmo formalisms

The electric field gradients (EFG) at the D. Li. N and O sites in the linear molecules LID, DF, DCN, DCCD. OCCF, N2, CO and NCCN have been rigorously evaluated with the inclusion of all integrals using four different semi-empirical SCFMO methods with a view to assess their suitability for EFG calculations. The methods chosen are the CNDO/2 and INDO methods of Pople, a method using explicitly orthogonalised AO's and distinguishing s and p orbitals in the valence shell due to Nanda and Narasimhan (NN-INDO) and a reparametrisation of the same using Clementi-Raimondi exponents. It is found that orbital exponents play a crucial role in semi-empirical EFG calculations. Use of explicitly orthogonalised basis sets as in the NN-INDO schemes is seen to improve the EFG values for the first-row atoms. A few comments are made on population-based methods for EFG calculations