[molpro-user] Full CI convergence problems with hydrogen

Lorenzo Lodi l.lodi at ucl.ac.uk
Tue Mar 20 12:06:36 GMT 2007

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
r = 0.8
rhf, WF, ELEC=2, SYM=1, SPIN=0

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

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