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# H0_BLOCKSIZE

> This parameter specifies the size of the H0 block of the Hamiltonian which is solved exactly. The n determinants with the lowest SCF energy are selected, and a submatrix of the Hamiltonian is formed using these determinants. This submatrix is used to accelerate convergence of the CI iterations in the OLSEN and MITRUSHENKOV iteration schemes, and also to find a good starting guess for the SEM method if GUESS_VECTOR is `H0_BLOCK`. Defaults to 400. Note that the program may change the given size for Ms=0 cases MS0 is TRUE) if it determines that the H0 block includes only one member of a pair of determinants related by time reversal symmetry. For very small block sizes, this could conceivably eliminate the entire H0 block; the program should print warnings if this occurs.

> - Type: integer
> - Default: 400

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