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International Journal of Quantum Chemistry

Invariance transformations and AGP optimization

Richard L. Lozes, Brian Weiner, Yngve Öhrn

Published Jun 19, 2009• 0 citations
Publisher

Abstract

Optimization of the energy expectation for an antisymmetrized geminal power (AGP) wavefunction is discussed utilizing the antisymmetric bilinear (symplectic) form of the geminal.
DOI
10.1002/qua.560200813

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We present a powerful iterative algorithm for optimizing the geminal coefficients of an antisymmetrized geminal power (AGP) wavefunction. The algorithm is based on the lowest eigenvector of a matrix closely related to the Hessian of the problem. This matrix can be derived either by using Euler's theorem or by utilizing a unitary group approach. Two important features of the scheme are that it will always converge towards a minimum and that in the neighborhood of a minimum it is comparable to a quadratically convergent Newton-Raphson method.
We investigated some variational methods to compute a wavefunction based on antisymmetric product of geminals (APG). The Waring decomposition on the APG wavefunction leads a finite sum of antisymmetrized geminal power (AGP) wavefunctions, each for which the variational principle can be applied. We call this as AGP-CI method which provides a variational solution of the APG wavefunction efficiently. However, number of AGP wavefunctions in the exact AGP-CI formalism become exponentially large in case of many-electron systems. Therefore, we also investigate the low-rank APG wavefunction, in which the coefficient matrices of geminals are factorized by the Schur decomposition. Interestingly, only a few non-zero eigenvalues (up to half number of electrons) were found from the Schur decomposition on the APG wavefunction. We developed some methods to approximate the APG wavefunction by lowering the ranks of coefficient matrices of geminals, and demonstrate their performance. Our new geminal method based on the low-rank decomposition can drastically reduce the number of variational parameters, although no efficient formalism has been elaborated so far, due to the difficulty of optimization caused by the inability to compute analytical derivatives.
Abstract The properties of the n‐electron wave functions equation image with symplectic symmetries have been discussed. These functions are built up with two kinds of geminals. The well‐known antisymmetrized geminal power (AGP) function is only a special case of them. The first‐ and second‐order reduced density matrices and the natural expansions of these functions have been given, and the uniform expressions for system energies in the states corresponding to these functions have been deduced. It has been shown that the special case equation image is probably another consistent ground state for the particle‐hole propegator or random‐phase approximation (RPA) besides the AGP function.