[molpro-user] Full CI convergence problems with hydrogen

[Tatiana Korona]

You should try to change default thresholds e.g. for integrals.

Tatiana Korona

On Tue, 20 Mar 2007, Lorenzo Lodi wrote:

> I was trying as a test some FCI calculation for the hydrogen molecule (I am 
> using molpro v2002.6), but when I try with the aug-cc-pV5Z basis set I have 
> convergence problems.
> This is the input file:
>
> memory, 80, M
> basis=aug-cc-pV5Z
> r = 0.8
> geometry={h1;h2,h1,r}
> rhf, WF, ELEC=2, SYM=1, SPIN=0
> FCI
>
> I get a FCI energy of -3.057081385992 hartrees, which is wrong.
> On the other hand, if I try with a smaller basis set like aug-cc-pVQZ I get 
> -1.082735905256 hartrees, which I belive is the correct result for this basis 
> set.
> For values of r greater than 0.9 bohrs everything is OK even with the A5Z 
> basis set.
> Incidentally, I've also tried other methods like MRCI or CCSD but I always 
> get some errors for some combinations of bond lenghts/basis set.
> Is there anything that can be done about this?
>
> N.B.: the calulation FCI(A5Z) takes about 5min of a PC.
>
> Thank you,
>
> Lorenzo Lodi
>