Mr. Rebelo,

You should use only CISD.

So by using the CISD alone, I can just perform single and double substitutions on the ground state. I'm supposing that there is not a way to perform the single and double substitutions on excited states...Is that it?

Mr. Rebelo,

The answer to your more recent question is: Not, it is not. There is a way, using namely only 'CISD'. The correction to the energy is obtained as an output-data (below).

A general descriotion of CIS(D) computations by Gaussian can be found, here: http://gaussian.com/cis/

You have obtained as an output the followings:

Example of H2O:

#p cisd/sto-3g test polar=enonly iop1=synch..||0,1|O,0,0.,0.116374,0. |H,0,0.740783,-0.4655,0.|H,0,-0.740783,-0.465495,0.........|State=1-A'|HF=-74.9602903|MP2=-74.9945723|MP3=-75.003784|CISD=-75.0072792|RMSD=1.875e-010|Dipole=-0.0000023,-0.6494454,0.|Polar=4.67 68642,-0.0000172,2.0692975,0.,0.,0.0458229|HyperPolar=0.0000267,-5.064

765,-0.0008307,-3.4151158,0.0000361,-0.0000459,-0.0001454,-0.0000095,0 .0796719,0.|Quadrupole=0.6200528,-0.0639943,-0.5560585,-0.0000025,0.,0.|PG=CS [SG(H2O1)]||@

The perturbative correction in CISD involves only treatment of doubles space. The triples space is not treated by strict perturbative theory. CISD reproduces systems with well separate in energy terms states. It it is not, however, it is better to use only CIS. In addition, you could use as well as CIS-MP2, where the treatment is via standard perturbation theory to both spaces.

You could pay attention to an example of H2O:

E(CISD) = -75.0072792 a.u.

E(CIS) = -74.4576737410 a.u.

E(CIS-MP2)= -74.5696057951 a.u.

CIS-MP2 perturbative correction on CIS state can be obtained using: CIS(root=X) iop(9/23=1)

The output is depending on X.

For example, H2O again:

***********************************************************************

2nd-order MP Correction to the CiSingles state 1

***********************************************************************

Calling MO disk program.

This state is a triplet.

CIS Exc. E: 0.424548019211 a.u. 11.55254 eV 107.32202 nm

CIS-E2 doubles : -0.192478563963E-01

T4(AAA)= -0.13141800D-35

T4(AAB)= -0.73078522D-02

Time for triples= 0.00 seconds.

CIS-E2 triples : -0.146157043506E-01

CIS-E2 Total : -0.338635607470E-01 E(CIS-MP2)= -74.5696057951

CIS-MP2 Exc. E: 0.424966466626 a.u. 11.56393 eV 107.21635 nm

Leave Link 914 at Wed Mar 15 17:34:52 2017, MaxMem= 33554432 cpu: 1.0

(Enter C:\g09w\l601.exe)

Copying SCF densities to generalized density rwf, IOpCl= 0 IROHF=0.

.........................................................................

...|0,1|O,0,0.,0.116374,0.|H,0,0.740783,-0.4655,0.|H,0,-0.740783,-0.465495,0.||State=1-A'|HF=-74.9602903|MP2=-74.9945723|RMSD=1.400e-0 12|PG=CS [SG(H2O1)]||@

Thank you very much for lhe clarification and suggestion.