[molpro-user] on Davidson correction

[Somnath BHOWMICK]

Dear Molpro users,

I am calculating excited states of lithium and fluorine clusters in MRCI theory. The active space is large enough (around 85000 CSFs) to describe the ground and few low-lying excited states. There is a large gain in correlation energy (about 7-8 eV) from the reference energy at the MRCI level. Moreover, the coefficient of the reference function in the MRCI wave function, i.e. c_n (as mentioned in https://www.molpro.net/info/2012.1/doc/manual/node310.html) is only around 0.94. This makes a huge gain in correlation energy (1 eV) in the Davidson correction term (for both fixed and rotated references) from the conventional MRCI energy. I was wondering if this is normal for most MRCI calculations or there is a limit of this energy difference or there exist some predefined conditions on the breakdown of Davidson correction.

Thank you in advance.

With very best of regards
Somnath Bhowmick
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