GERSH — Use Gershgorin method to compute bounds on the Fock matrix eigenvalues.
PSOLVE=PDM and PSOLVE=CEM require the upper and lower bounds of the Fock matrix eigenvalues. By default, the exact bounds are determined by a special solver but GERSH will force it the use of the Greshgorin theorem instead to find approximate bounds for these eigenvalues. (This method is also used in the event that the standard solver fails.) The main advanages of the Gershorgin method are that it is non-iterative, simple, and fast. The primary disavantage is that it only produces approximate bounds and so span a much larger range than the real bounds. Consequently, it gives a poorer initial guess for PDM and CEM, which then requires more purification to achieve convergence.
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