# Excited states using ORMAS flag in GAMESS

I am trying to calculate the excited states using single reference CISD on $$\ce{CH3OH},$$ using ORMAS' CI-X option in GAMESS. As a test example I am using the $$\ce{CH3OH}$$ that has 2 core, 7 occ. valence, 39 empty MOs (50 AOs, 48 MOs). I am unable to understand the $ormas flag in the following ormas-meoh-cisd.inp file: ! single reference CISD on CH3OH, using ORMAS' CI-X option. ! counting: 2 core, 7 occ. valence, 39 empty MOs (50 AOs, 48 MOs) ! the resulting CI-SD has 53,964 Sz=0 A' determinants ! $$contrl scftyp=rhf cityp=ormas runtyp=energy ispher=1$$end $$system mwords=50$$end $$basis gbasis=ccd$$end $$guess guess=huckel$$end $$scf dirscf=.true.$$end $$trans dirtrf=.true.$$end $$cidet group=Cs stsym=Ap ncore=2 nact=46 nels=14 nstate=3$$end $$ormas nspace=2 mstart(1)=3,10 mine(1)=12,0 maxe(1)=14,2$$end$data
Methanol...CISD...RHF/6-31G(d) geometry
Cs

H     1.0  -1.0616171503   0.8036449245   0.0000000000
O     8.0  -0.6870131482  -0.0653470836   0.0000000000
C     6.0   0.7093551399   0.0291827007   0.0000000000
H     1.0   1.0836641283   0.5408321444   0.8835398105
H     1.0   1.0975386849  -0.9797829903   0.0000000000
$end  The following keywords are used in the $ORMAS section:

nspace=2
mstart(1)=3,10
mine(1)=12,0
maxe(1)=14,2


I can understand the nspace = 2 means the active space is partitioned into doubly occupied + active and valence orbitals. However, I don't understand how the numbers pertinent to the rest of the flags in \$ORMAS are chosen. Any explanation would be really helpful and appreciated.

• It seems you may have accidentally made a second account. You can use this form to request that they be merged together.
– Tyberius
May 14 at 16:12

• nspace=n means the active space will be split into $$n$$ groups.
• mstart sets the starting orbital for each of the $$n$$ groups. In your case, the first group extends from orbital 3 to orbital 9, while the second is from orbital 10 to NCORE+NACT (the number of core and active space orbitals). They mention that in this only makes sense if you have used NORDER in the guess section (I suppose otherwise there is no guarantee the active space is contiguous or that it is right after the core).
• mine sets the minimum number of electrons for each group.
• maxe sets the maxium number of electrons for each group.