CASINO manual - Theory of Condensed Matter

CIS
• One cannot use basis functions containing g and higher basis functions with a pseudopotential
in g94.
• Obviously, gaussiantoqmc needs the molecular orbitals (MOs) produced by the calculation.
It gets these from the formatted checkpoint file which is produced by putting ‘Formcheck=(Basis,MO)’
in the route section of the gaussian job. Alternatively, it may be obtained
from the binary checkpoint file (.chk) using the formchk utility: see the gaussian manual.
• Many (but not all) of the IOps mentioned here are described on gaussian’s website at www.
gaussian.com/iops.htm.
• Performing a ‘HF test’ for an excited state: It is possible to get gaussian to output a breakdown
of the energy of a CIS excited state which may be compared with the results of a determinantonly
VMC run. The key to this is the density used to perform the population analysis and other
post-SCF calculations. By default, gaussian uses the density produced by the original SCF
run. To get the kinetic, nuclear-nuclear potential and electron-nuclear potential energies you
must tell it to use the one-particle CI density via ‘density=RhoCI’. You must also specify the
excited state that you are interested in via ‘Root=N’ in the CIS options. With all of this done
properly, gaussian produces some output like:
N-N= 6.9546274D+00 E-N=-2.3781323D+01 KE= 3.3771062D+00
(units of a.u.), which is hidden in the density analysis right at the end of the output.
• Some trouble has been encountered with the ‘Add=N’ option to CIS (which reads converged excited
states from the checkpoint file and then calculates N more). The IOp alternative which does
work is IOp(9/49=2) (use guess vectors from the checkpoint file) combined with IOp(9/39=N)
(make N additional guesses to those present).
• Using the ‘50–50’ option to CIS to calculate singlet and triplet excitations simultaneously can
cause problems. It appears best to do the singlet (‘singlets’) and triplet (‘triplets’) calculations
separately.
• For QMC, we want the complete CIS expansion. gaussian may be persuaded to output all
excited states with coefficients > 10 −N by using IOp(9/40=N). Typically N = 5 is good
enough. gaussiantoqmc outputs the sum of the square of the coefficients so that the user
can see how complete the wave function is. (Standard gaussian output has the coefficients
normalized so that the sum of their squares for a complete expansion would be unity.)
CISD
Although gaussiantoqmc cannot read a CISD wave function it might be worth mentioning that
IOp(9/6)=N is equivalent to MAXCYCLE=N for such a calculation.
CASSCF
• As described in Foresman and Frisch’s book [22], getting CASSCF to converge for a singlet state
is difficult. The following procedure normally works:
1. Run a ROHF calculation for the lowest triplet state of the system and save the checkpoint
file.
2. Run CASSCF for the second triplet state (‘Nroot=2’) taking the initial guess from the
checkpoint file. (gaussian calculates the first and second triplets but converges on the
second.)
3. Run CASSCF for the first triplet (‘Nroot=1’) taking the initial guess from the checkpoint
file.
4. Run CASSCF for the singlet excited state (‘Nroot=2’) taking the initial guess from the
triplet checkpoint file.
5. Finally, run CASSCF for the singlet ground state (‘Nroot=1’).
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