# [molpro-user] problem with MRCI breaking degeneracy of 1-Delta state

Irikura, Karl K. Dr. (Fed) karl.irikura at nist.gov
Tue May 22 23:10:59 CEST 2018

```Dear MOLPRO experts,

For the 1-Delta state of oxygen (O2), I get nice degeneracy with CASSCF but an (incorrect) energy splitting from MRCI.  There is no problem if I use NOSYM, but it will take long in a bigger system.  What is my mistake?

--------problematic case (D2h)----------
***, O2 singlet states
! casscf(6,4), state-averaged orbitals
!
memory,1000,m
basis=6-311G*

angstrom
geometry={
O
O,1,1.2156
}

{rhf
occ,3,1,1,0,2,1,1,0;
closed,3,1,1,0,2,0,0,0;
wf,16,4,2;
}

{multi
occ,   3,1,1,0, 2,1,1,0;
frozen,1,0,0,0, 1,0,0,0;
closed,3,0,0,0, 2,0,0,0;
wf,16,4,2; state,1; lquant,0;
wf,16,4,0; state,1; lquant,2;
wf,16,1,0; state,2; lquant,2,0;
expec2,LZZ;
}

cas = energy

! MRCISD calculations
{ci; wf,16,4,2; save,4042.1; }
mrci = energy
{ci; wf,16,4,0; save,4040.1; }
mrci(2) = energy
{ci; wf,16,1,0; state,2; save,4010.1; }
mrci(3) = energy

! convert absolute hartree to relative wavenumber
do I=4,1,-1
cas(I) = (cas(I) - cas(1)) * TOCM
mrci(I) = (mrci(I) - mrci(1)) * TOCM
enddo

! print table of relative energies
table, cas, mrci
title, Relative energies in cm-1
ftyp,f,f
digits,1,1

output:
Relative energies in cm-1
CAS      MRCI
0.0       0.0
6056.6    7751.0
6056.6    7620.8
11466.1   12890.0

-------no problem (C1)-------------
***, O2 singlet states
! casscf(6,4), state-averaged orbitals
!
memory,1000,m
basis=6-311G*

NOSYM
angstrom
geometry={
O
O,1,1.2156
}

{rhf
occ,9;
closed,7;
wf,16,1,2;
}

{multi
occ,9;
frozen,2;
closed,5;
wf,16,1,2; state,1; lquant,0;
wf,16,1,0; state,3; lquant,2,2,0;
expec2,LZZ;
}

cas = energy

! MRCISD calculations
{ci; wf,16,1,2; save,4012.1; }
mrci = energy
{ci; wf,16,1,0; state,3; save,4010.1; }
mrci(2) = energy

! convert absolute hartree to relative wavenumber
do I=4,1,-1
cas(I) = (cas(I) - cas(1)) * TOCM
mrci(I) = (mrci(I) - mrci(1)) * TOCM
enddo

! print table of relative energies
table, cas, mrci
title, Relative energies in cm-1
ftyp,f,f
digits,1,1

output:
Relative energies in cm-1
CAS      MRCI
0.0       0.0
6056.6    7564.9
6056.6    7564.9
11466.1   12822.4

------------------------

Thanks for any help!
Karl

-------------------------------------------------------------------
Dr. Karl K. Irikura
National Institute of Standards and Technology
100 Bureau Drive, mail stop 8320
Gaithersburg, MD 20899-8320
voice:  301-975-2510   fax:  301-975-3670
email:  karl.irikura at nist.gov<mailto:karl.irikura at nist.gov>
http://www.nist.gov/mml/csd/informatics_research/index.cfm
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