Debug complex Lanczos mode for systems with exchange and/or interall interactions #62
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There is a possible bug when a trial wavefunction with complex variational parameters is used for Lanczos calculations in systems with exchange and/or interall interactions.
For such systems, the energy obtained using Lanczos method becomes lower than that only using VMC but it is not as low as expected. The variance also sometimes becomes larger than that only for VMC results. ( Lowering the variance is not guaranteed in the Lanczos procedure. ) The Lanczos calculation using complex variational wavefunction with 1 sublattice structure in the Heisenberg model is an example.
The modified code in this PR seems to work well at least for the example mentioned above.