[molpro-user] dissociating H2 molecule?

[H. -J. Werner]

1.) In order to get the symmetry broken (spin contaminated) solution, you
must (i) do the calculation in lower symmetry (x,y if z is the bond axis),
and (ii) use symmetry broken starting orbitals. The latter can be 
obtained by first doing a rhf calculation for the triplet, and then
rotating the two open-shell orbitals by 45 degrees (using ROTATE).
This input does the job:

geometry={x,y;h1;h2,h1,r}
r=4
hf;wf,spin=2
uhf;wf,spin=0;rotate,1.1,2.1,45


2.) The only correlation method in Molpro for UHF references is UMP2,
but this is not a recommended method to use (due to well known
spin-contamination problems).

H.-J. Werner



On Fr, 08 Jul 2005, Julius T. Su wrote:

>Hello,
>
>  I had two questions -- (1) how can I make a UHF wavefunction corresponding to dissociating H2 and (2) what correlation methods would be accurate for this case?
>
>  For (1), I can handle the triplet case without any problem, just by specifying the right symmetry orbitals to be open-shell.  However this doesn't work for the singlet case, either with or without symmetry.  Could someone show me how to do this?  (I know it's not an s^2 eigenfunction, but that should still be all right with molpro, right?) 
>
>  For (2), is there a CCSD(T) method that starts from a UHF wavefunction?  Is it reasonable to expect an RHF based method to handle this case well?  CR-CCSD(T) in GAMESS handles H+H well, do the UCCSD(T) methods in molpro work similarly? 
>
>Thanks,
>
>Julius

-- 
Prof. Hans-Joachim Werner
Institute for Theoretical Chemistry
University of Stuttgart
Pfaffenwaldring 55
D-70569 Stuttgart, Germany
Tel.: (0049) 711 / 685 4400
Fax.: (0049) 711 / 685 4442
e-mail: werner at theochem.uni-stuttgart.de