[molpro-user] Why the MCSCF energy is lower than Full-CI?

[Kirk Peterson]

Hi,

I believe the occ directive in your FCI calculation should be the same as the HF reference, i.e., occ,2;   or just
leave out both the occ and core directives and take the defaults. For me at least with 2009.2, this then
yields the same energy for FCI and CASSCF. 

-Kirk

On Feb 13, 2010, at 7:28 AM, cong.wang wrote:

> Dear everyone,
> 
>   Hello,
> 
>    Excuse me, I did a HF, CASSCF(4,8), and Full-CI calculations for
> H4 square molecule with 6-31G basis set on molpro 2009. The CASSCF
> energy is lower than FCI by 0.02 a.u., what would be the explanation
> for that? Should the CASSCF(4,8) and FCI be equivalent  in this case?
> 
>  Here are the input and output files
> 
> Thank you very much in advance
> Best regards
> Sincerely,
> Cong Wang
> 
> Ph. D. Student
> Department of Chemistry
> Laboratory for Instruction in Swedish
> University of Helsinki
> A.I. Virtanens plats 1
> P.O. Box 55
> FI-00014 University of Helsinki
> FINLAND
> 
>  ***,h4     !A title
> symmetry,nosym
> geomtyp=xyz
> geometry={
> 4
> title
> H     0.  0.     0.
> H     0.  0.     0.5
> H     0.  0.5    0.
> H     0.  0.5   0.5
> }
> 
> basis={
> !
> ! HYDROGEN       (4s) -> [2s]
> s, H , 18.7311370, 2.8253937, 0.6401217, 0.1612778
> c, 1.3, 0.03349460, 0.23472695, 0.81375733
> c, 4.4, 1
> }
> 
> 
> hf                        !closed-shell scf
> {fci;occ,8;core,0;wf,4,1}
> {multi;occ,8}
> 
> *******************