I asked whether it is possible to calculate and optimize the ground state of a system with TDHF. The answer is no. Like CIS, TDHF does not provide a correction to the ground state energy. More important I was warned that the TDHF geometry optimization for excited states in G98 is an approximation and should not be used. The option has been removed in G03.

One comment of mine to the statement that TDDFT is superior to TDHF: I am aware that TDDFT is quite successful in general. Unfortunately, the only excited state that is currently of interest to me (the 1Bu excited state of polyenes) is not well reproduced, especially with increasing size and conjugation length, which is exactly what I need to investigate. TDHF, in contrast, gives numbers similar to CASMP2. Therefore, it is bad news indeed that the geometries can not be optimized at this level of theory.

The original question and the answers follow below:

Hello,the time-dependent Hartree-Fock method is described as a method for calculating excited states. I would like to know whether one can also optimize the ground state including correlation with TDHF. In other words, if I use the keywords "fopt" and "nroot=0" in Gaussian what am I calculating? I know that CIS does not make a correction to the ground state because of Brillouin's theorem but I am not sure what TDHF includes exactly. The reason why I am considering this is that I would like to compare the excitet state and the ground state geometries on equal footing. Would it be better to compare the excited state TDHF geometry to the HF ground state geometry? Thanks in advance, Ulrike Salzner

Dear Ulrike, in fact TDHF is not defined for the ground state but there is a close analogy between CIS and TDHF (expanded in singles only, CIS equations can be derived from the non-Hermitian TDHF problem by neglecting the so-called B-matrix). >From that I would argue that the ground state analogue of TDHF is just HF. Regards Stefan _________________________________________________________ Prof. Dr. Stefan Grimme Organisch-Chemisches Institut (Abt. Theoretische Chemie) Westfaelische Wilhelms-Universitaet, Corrensstrasse 40 D-48149 Muenster, Tel (+49)-251-83 36512/33241/36515(Fax)

Ulrike, Just like CIS, TDHF does not improve the ground state. So correct comparison would be TDHF optimized excited state and HF optimized ground state. In fact, TDHF only differs from CIS in that it has nonzero V-O block in hamiltonian (and thus, in transition density) matrix. In g98 this block is missing from the routine evaluating gradients, so excited state optimization with analytical gradients gives incorrect (approximate) results. That is why in g03 this option is blocked. In Turbomol TDHF opt. is coded fine. To optimize the excited state at TDHF level you need to do Opt=Numer, which is much slower. Please keep in mind that TDHF is inferior to TDDFT at the same computational cost. Hope this helps, Artem ________________________________________________________ __ ___________ Artem.Masunov^at^LANL.gov / \ / __ __ \ www.t12.lanl.gov/home/amasunov / \/\ \ \ \ \ \ 505.665.2635, Fax:505.665.3909 / /\ \ \ \ \ \ \ \ Theoretical Division, MS B268 / ____ \ \ \ \ \ \ \ Los Alamos National Lab /__/\ _/\ _\ \ _\ \ _\ \ _\ Los Alamos NM 87545 \ _\/ \/__/\ __/\ __/\ __/ ____________________________ Dr. Salzner, G98/03 does not have gradients implement analytic gradients for TDHF or TDDFT so optimizations need to be performed with only energy values OPT=(EF,EnOnly) and a symbolic Z-matrix. But it can be done for medium to small systems. Or at least a few degrees of freedom. The TDHF and CIS methods both use the HF solution as the reference and neither of them improve on the ground state description. In the sense that neither the Excited state or the Ground state is really correlated it has been our experience that structures in CIS are similar in quality to HF although often a slightly larger basis is needed. To go beyond this you might want to consider SACCI which is in G03 or CASSCF, both of which can include correlation, dynamic vs. static, and treat ground and excited state on a equal footing.

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