Unitary group approach

Evaluation in the Unitary Group Approach to the Electron Correlation Problem. 

Active Space Self-Consistent Field Method, Implemented with a Split Graph Unitary Group Approach. 

Based on the Graphical Unitary Group Approach. 2. Single and Double Replacements From any Set of Reference Configurations. 

A. Koslowski, M. E. Beck, and W. Thiel. Implementation of a general multireference configuration interaction procedure with analytic gradients in a semiempirical context using the graphical unitary group approach, J. Comput. Chem., 24 714-726 (2003). 

X. Li, Q. Zhang, Int. J. Quant. Chem. XXXVI, 599 (1989). Bonded Tableau Unitary Group Approach to the Many-Electron Correlation Problem. 

Configuration Interaction Using the Graphical Unitary Group Approach and Relativistic Core Potential and Spin—Orbit Operators. 

Eqs.(6) and (7) imply that the basis in the model space is formed by spin-adapted combinations of Slater determinants. This way of constructing the model space is specific for the unitary group approach to the theory of TV-electron systems [15, 16]. 

We overview our valence bond (VB) approach to the ir-electron Pariser-Parr-Pople (PPP) model Hamiltonians referred to sis the PPP-VB method. It is based on the concept of overlap enhanced atomic orbitals (OEAOs) that characterizes modern ab initio VB methods and employs the techniques afforded by the Clifford algebra unitary group approach (CAUGA) to carry out actual computations. We present a sample of previous results, sis well sis some new ones, to illustrate the ability of the PPP-VB method to provide a highly correlated description of the ir-electron PPP model systems, while relying on conceptusilly very simple wave functions that involve only a few covalent structures. 

At the ab-initio level, the most obvious possibility is offered by CAS SCF or CAS FCI (i.e., Cl within the CAS or, equivalently, CAS SCF without the orbital reoptimization based on RHF orbitals, cf. [33, 34]) wave functions based on the smallest possible active-space that warrants the correct description of the dissociation channel at hand. This option was also suggested by Stolarczyk [29], although we are not aware of any concrete implementation. Our testing proved to be very encouraging [33, 34], particularly for open shell systems, in which case we employed the spin-adapted CCSD based on the unitary group approach (UGA) [16, 36]. Even in the case of triple bond breaking, the applicability of the CCSD approximation can be significantly extended, as will be shown in Sect. 4. Most recently, we have explored the MR CISD wave function as an external source, as described in the next section. 

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