CASINO manual - Theory of Condensed Matter
CASINO manual - Theory of Condensed Matter
CASINO manual - Theory of Condensed Matter
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• By default, gaussian uses spin configurations (combinations <strong>of</strong> Slater determinants) in a<br />
CASSCF calculation. It is best to converge the CASSCF state that you want using this option.<br />
However, for input to the QMC code, the wave function must be in terms <strong>of</strong> Slater determinants.<br />
In principle, the ‘SlaterDet’ option to CASSCF will do this, but I never succeeded in getting it<br />
to work. Instead, specifying IOp(4/21=10) does the trick, as does IOp(4/46=3).<br />
• For large CASSCF calculations on the alpha cluster Columbus, the diagonalization method must<br />
be changed by specifying IOp(5/51=1).<br />
• In large CASSCF calculations where very many determinants are involved, gaussian currently<br />
only prints the first fifty determinants in the expansion (those with the largest coefficients).<br />
The significance <strong>of</strong> the truncation may be judged by looking at the sum <strong>of</strong> the squares <strong>of</strong> the<br />
coefficients that gaussiantoqmc outputs when it reads the wave function. Unfortunately, this<br />
truncation prevents a full ‘HF test’ (i.e., running the wave function in VMC without a Jastrow<br />
factor and checking that the result agrees with that <strong>of</strong> gaussian), but the energy returned by<br />
such a test should be above that reported by gaussian.<br />
• As well as this truncation to fifty determinants, gaussian has a formatting error which means<br />
that if the index number <strong>of</strong> a determinant is greater than 99,999 then it is replaced by stars and<br />
is thus useless. gaussiantoqmc deals with this by simply throwing away such configurations<br />
which further truncates the expansion.<br />
• gaussian switches to direct mode for large CASSCF calculations and in doing so automatically<br />
stops printing the definitions <strong>of</strong> the Slater determinants used in the calculation. In order to reconstruct<br />
the wave function we do <strong>of</strong> course need to know what the Slater determinants are. gaussian<br />
may be persuaded to print them by using IOp(4/46=3) IOp(4/21=10) IOp(4/21=100).<br />
The first two <strong>of</strong> these both tell gaussian to use Slater determinants (I specified both to be on<br />
the safe side) and the last one in theory tells it to ‘just print the configurations’ although in fact<br />
it still proceeds to do the calculation as well.<br />
• Restarting a CASSCF calculation from a previously converged run fails when symmetry is<br />
switched on. Use ‘Nosymm’ to avoid this problem.<br />
TD-HF and TD-DFT<br />
As far as converting the resulting wave function for use in casino is concerned, these two methods<br />
are no different to CIS apart from the issue <strong>of</strong> normalization. In CIS, the default output is normalized<br />
so that the sum <strong>of</strong> the squares <strong>of</strong> the coefficients is equal to unity. In a TD-HF or TD-DFT calculation<br />
(which involves solving a non-Hermitian eigenvalue problem) a different scheme is used which<br />
essentially means that the sum <strong>of</strong> the squares <strong>of</strong> the coefficients is arbitrary.<br />
It should also be noted that gaussian cannot do gradients within TD-DFT yet and so cannot relax<br />
excited states.<br />
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