Orbital transformations and configurational transformations of electronic wavefunctions

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Transformations among molecular orbitals are often expedient or illuminating, and sometimes essential in quantum chemical contexts. In order to express the many-electron wavefunction in terms of the corresponding transformed configurations, full CI calculations used to be repeated in the transformed orbital basis. The configurational transformations can however be obtained directly, as shown by Malmqvist, by a factorization into single orbital transformations. In the present paper, a direct transformation method is presented that is based on the factorization of orbital transformations in terms of Jacobi rotations. Compared to the repetition of a CI calculation, both direct re-expansion methods drastically reduce the computational effort and increase the numerical accuracy. They are, moreover, applicable to wavefunctions whose original construction is not accessible.