Subject: Re: CCL:Why aren't DFT methods suitable for long distance interactions?

From: Lou Noodleman <[email-masked]>
Date: Thu, 14 Oct 1999 12:16:02 -0700 (PDT)

Dear Yubo Fan and  CCL,
DFT, especially in the form of gradient corrected potentials has
proved suitable for many applications involving H-bonds. This includes
water clusters, H-bond donor-acceptor systems, for metal ions in
solution (redox processes)(see Li et al Inorg. Chem. 1996, 35, 4694-4702),
 and for transition
metal complexes in proteins (metalloproteins)(Li et al., Inorg. chem. 1999, 38,929-939, Konecny et al., Inorg. Chem. 1999, 38, 940-950). Overall, fairly polar,
dipolar, or especially charged H bonds look to be well-represented in most
cases. One Ref. is Pudzianowski, J.Phys.Chem.1996, 100, 4781-4789. Another
example is the recent discussion and controversy in the literature over
low-barrier hydrogen bonds (LBHB) or more generally, short strong hydrogen
bonds (SSHB). Hydrogen bond energies are quite similar for density functional
(BLYP) versus ab initio MP2 results (for substituted formate-formic acid dimers)
(see Kumar and McAlister J.A.C.S. 1998, 120, 3159). Whatever the outcome of
this controversy, DFT methods look suitable for analyzing these issues.
 DFT does have
some specific deficiencies for delocalized 3-electron bonds, which are too
stable. So, for example, the ground state of the [(H2O)2]+ radical cation is
incorrectly found as the "hemibond" structure rather than the correct proton
transferred OH-H3O+ isomer.(Sodupe et al., J.Phys.Chem.A 1999, 103, 166-170.)
This corresponds to a problem that my coworkers and I identified in DFT long ago
(Noodleman, Post, Baerends Chem. Phys. 1982, 64, 159. This can be partially
remedied by looking for broken symmetry DFT solutions with charge asymmetry.)
 Overall, however, this
is an atypical problem. In terms of H-bonded systems, it appears that quite weak H-bonds are difficult for DFT. Weak long range Van der Waals interactions 
(dispersion energies) are also evidently not well treated by standard DFT
 methods from what I have seen (as might be expected).
Louis Noodleman
Department of Molecular Biology
The Scripps Research Institute, TPC15
La Jolla, CA 92037
.