[molpro-user] CASSCF calculation

Jeremy Merritt merritjm at unc.edu
Mon Oct 31 16:52:20 GMT 2005

```Dear molpro users.
I am new to multi-reference calculations so I hoped I could get some help on a problem.

I am trying to calculate the 3 long range PES is Cs symmetry (1A' ,2A', 1A") for the Gallium (2P) + HCN interaction neglecting spin-orbit.  The ground electronic state of Ga-HCN (van der waals complex) should be doublet sigma, where as HCN-Ga should be doublet pi.  I have confirmed this with single reference calculations.  So I would like to do a CASSCF(1,3) calculation with all three states weighted equally.  I would like to take this to higher levels (MRCI, or MRACPF etc..) but my question starts with the CASSCF.

Here is my input:
gprint,civectors
mH=1.00794
mC=12.0
mN = 14.01
rCH = 1.06
rCN = 1.17
R = 3.5
theta = 0.0
rCM = ((mN*rCN)-(mH*rCH))/(mH+mC+mN)

geometry= {Z,ANGSTROM,
C,
X, C, 1.0,
N, C, rCN, X, 90,
H, C, rCH, X, 90, N, 180,
X2, C, rCM, X, 90, N, 0,
Ga, X2, R, N, theta, X, 0,
}
basis =cc-pVDZ
i=0;
Do theta=0.0,180.0,180.0,
Do R=6.5,6.5,-0.2,
i=i+1
T(i)=theta
Rs(i)=R
rhf
wf,45,2,1;
multi,
occ,19,6
closed,17,5
wf,45,1,1;STATE,2;wf,45,2,1;

Ea(i)=energy(1)
Eb(i)=energy(2)
Ec(i)=energy(3)

the pertinent output:
T    RS         EA             EB             EC
0.0   6.5   -2016.070571   -2016.069385   -2016.070571
180.0   6.5   -2016.071635   -2016.069526   -2016.069526
so one can see at 0 degrees (HCN-Ga) Ea & Ec are degenerate so they are the two Pi states and Eb is the sigma state.  Also the Pi is lower than the sigma which is correct.
but at 180 degrees (Ga-HCN) now Eb & Ec are degenerate, so they are the Pi states.  Ea is the sigma state and it is predicted to be lower in energy which is correct.
For simple linear geometries it is easy to pick out the switching of the labeling, but when the states cross at nonlinear geometries i don't know how to maintain a consistent labeling.

I've tried looking at the CI vectors:

(HCN-Ga)
CI vector for state symmetry 1
------------------------------

0+ 0      1.0000000   0.0000000
+0 0      0.0000000   1.0000000

TOTAL ENERGIES                     -2016.07057096 -2016.06938539

CI vector for state symmetry 2
------------------------------

00 +      1.0000000

TOTAL ENERGIES                     -2016.07057096

(Ga-HCN)

CI vector for state symmetry 1
------------------------------

0+ 0      1.0000000   0.0000000
+0 0      0.0000000   1.0000000

TOTAL ENERGIES                     -2016.07163470 -2016.06952643

CI vector for state symmetry 2
------------------------------

00 +      1.0000000

TOTAL ENERGIES                     -2016.06952643

but they don't tell me much.

Any help would be appreciated.
___________________________
Jeremy Merritt
Graduate Student, Roger Miller lab
University of North Carolina
Chapel Hill

Voice:919-962-1579
Fax:919-843-6041
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