Second-Order Hyperpolarizabilty and Electron Correlation Effect of 2, 4-Bis（amino） -1, 3 -Cyclobutanedione

The optimized geometry and vibrational frequencies of 2, 4-bis (amino)-1,3 -cydobutanedione (NSQ) were calculated at ab initio/6- 3iG^** level. The linear polarizability α and second-order hyperpolarizability γ have been studied respectively by coupled-perturbed Hartree-Fork (CPHF) and finite field (FF)methods employing the same basis set 4- 3iG+pd/p. These two methods give similar results at RHF level. As compared with the HF results, the isotropic averages <α> and <γ> calctilated by MP2 electron-correlation correction increase 9% and 78%, respectively. The studies for the excitations show that NSQ consists of the donor-acceptor-donor structure. An interpretation based on the three-state model is given.