Singlet state computational chemistry

The key to get a diabatic electronic state is a strict constraint i.e. keep local symmetry elements invariant. For ethylene, let us start from the cis con-former case. The nuclear geometry of the attractor must be on the (y,z)-plane according to Fig.l. The reaction coordinate must be the dis-rotatory displacement. Due to the nature of the LCAO-MO model in quantum computing chemistry, the closed shell filling of the HOMO must change into a closed shell of the LUMO beyond 0=n/4. The symmetry of the diabatic wave function is hence respected. Mutatis mutandis, the trans conformer wave function before n/4 corresponds to a double filling of the LUMO beyond the n/4 point on fills the HOMO twice. At n/4 there is the diradical singlet and triplet base wavefunctions. 

Bill Hase received his Ph.D. in chemistry in 1970, working in the research area of experimental physical chemistry under the direction of John W. Simons at New Mexico State University. His research included studies of the methylene singlet-triplet energy gap and of the unim-olecular decomposition of vibrationally excited alkane and alkylsilane molecules prepared by chemical activation. His career as a computational chemist began during his postdoctoral work with Don Bunker at the University of California, Irvine. In 1973 he joined the Chemistry Department at Wayne State University, where he remained until 2004, when he assumed the Robert A. Welch Chair in Chemistry at Texas Tech University. He remembers that his hrst computational chemistry classical trajectory computer program was written in assembly language and run on a PDP-10. 

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