Semiempirical time-dependent unrestricted Hartree–Fock approach for the determination of molecular linear and nonlinear optical properties

The time-dependent unrestricted Hartree–Fock (TDUHF) iterative procedure has been implemented at the semiempirical level in the MOPAC 2000 package to determine the (non)linear optical responses of open-shell systems. In addition to providing the dynamic quantities, this procedure enables the decomposition of the total (hyper)polarizabilities into their spin contributions. Its application to reference molecules using AM1 level is reported to illustrate the capabilities of the method compared to ab initio and experimental results. Although for small systems the semi-empirical scheme fails in reproducing the highly-correlated ab initio values, the TDUHF/AM1 approach appears as an efficient tool for studying larger conjugated systems.