From keith@kbw350.chem.yale.edu Thu Jul 18 00:18:24 1996 Received: from kbw350.chem.yale.edu for keith@kbw350.chem.yale.edu by www.ccl.net (8.7.5/950822.1) id XAA20002; Wed, 17 Jul 1996 23:18:48 -0400 (EDT) Received: by kbw350.chem.yale.edu (AIX 3.2/UCB 5.64/4.03) id AA22500; Wed, 17 Jul 1996 23:14:59 -0400 Date: Wed, 17 Jul 1996 23:14:59 -0400 From: keith@kbw350.chem.yale.edu (Todd Keith) Message-Id: <9607180314.AA22500@kbw350.chem.yale.edu> To: chemistry@www.ccl.net Subject: SCIPCM in G94 Re: > I have been having problems getting geometries (and in some cases SCF's) > to converge when using the scrf=scipcm option in G94. The higher > the dielectric constant, the more problem there seems to be. Does > anyone have any suggestions on how this problem might be cured? I > have already started with prior hessians and the molecules are simple, > so the problem seems to be the inclusion of solvent effects. > > Thanks in advance, John > This is one of a many such complaints I've seen on the CCL, so a reply (possibly long-winded) to the list might be helpful ... The problem is probably due to the multi-center surface integration procedure which, unfortunately, is used by default for SCI-PCM in G94. Often, a far superior procedure is to use the non- default single center surface integration procedure: when used appropriately, it is much faster and the surface integrals much more accurate than the default procedure. Geometry optimizations are much more stable using the single center method (a conseqence of the more accurate surface integrals). To invoke the single center integration procedure, one must enter some additional input on the input line which specifies the solvent's dielectric constant. Here is the format and a partial description of the general SCI-PCM input line for G94: Epsilon Cont NPhi NTheta ISurf Epsilon = Dielectric constant of solvent (for example: 78.3 for water). Cont = Value of isodensity surface in atomic units. 0.0004 au is recommended and the default. Molecular volumes defined by 0.0004 isodensity surfaces consistently agree fairly well with values inferred from corresponding measured pure liquid molar volumes or partial liquid molar volumes, which suggests that solute cavities by such isodensity surfaces are physically reasonable. NPhi = Number of equally spaced phi values for numerical surface integration, or total number of points in a Special grid. Number is per origin. Default is Special 302. Legitimate Special grids are currently: 110 146 194 302 434 590 770 and 974. NTheta = Number of Gauss-Legendre theta values for numerical surface integration, or 1 to use a Special grid, which is the default. If using Gauss-Legendre, this number should be about half of NPhi. ISurf = Whether to do surface integrals numerically using a single center (2) - the center of nuclear charge - or piecewise, using a multi-center method (3). 3 is, in principle, more generally applicable than 2 and is the default, but (2) is much faster and more accurate when applicable, which is often. (2) is applicable when, for every point on the cavity surface, the projection of the surface normal vector onto the corresponding unit vector from the single center is greater than zero. The SCI-PCM solute geometry optimizations require reasonably accurate surface integrals to work well. The accuracy of the SCI-PCM surface integrations can be monitored by the calculated total flux of the nuclear electric field thru the cavity surface, which should equal 4*Pi*Qnuc, where Qnuc is the total nuclear charge within the cavity. Another measure of accuracy is the total surface polarization charge, Qpol, as compared to the net charge of the cavity, Qcav, which is (1/(4*Pi)) times the net flux of the solute electric field through the cavity surface. If things are done correctly then: Qpol=-(1-1/Epsilon)*Qcav. I would strongly recommend using the single center integration procedure by default. As implied above, the G94 SCI-PCM implementation calculates useful independent measures of the accuracy of the SCI-PCM surface integrals. Unfortunately, the values of these quantities (along with other useful info like the cavity volume and surface area) are not printed out by default, but can only be seen (for every SCF iteration particularly the final one) by requesting #P and IOp(5/33=2), which also causes the A LOT of other printing in the .log file (Fock matrices, density matrices, etc.). Specifying #P alone will cause the interesting quantities to be printed out for the first SCF iteration, which may be adequate to determine whether the surface integrals are sufficiently accurate to proceed. So, if one uses the single center procedure, the main question is how many integration points to use. Here is a rough! guide of molecular size versus total number of points to use: 1 to 6 atoms --> 434 points (Special grid: NPhi = 434, NTheta = 1) 7 to 11 atoms --> 590 points (Special grid: NPhi = 590, NTheta = 1) 12 to 15 atoms --> 770 points (Special grid: NPhi = 770, NTheta = 1) 16 to 20 atoms --> 974 points (Special grid: NPhi = 974, NTheta = 1) > 20 atoms --> use equally spaced Phi, Gauss-Legendre theta with the total number of points being at least NAtoms*100 (e.g., for 1800 total points use NPhi = 60 and NTheta = 30). Of course, the actual shape of the solute molecule might dictate the need for more or less points, or possibly, for very large or very complex solutes the complete failure of the single center method. Here is an example input file for a HF/6-31G* SCI-PCM geometry optimization of N,N-dimethyl acetamide in acetonitrile (Epsilon = 35.9): #P HF/6-31+G* Opt SCRF=SCIPCM N,N dma HF/6-31+G* opt SCI-PCM e=35.9, Cont=0.0004, 770 pts, single cent 0 1 Z-Matrix, molecule specification 35.9 0.0004 770 1 2 Note that the use of the default multi-center procedure in the above case would require many more integration points (like 1500) and would thus be relatively slow and the optimization may not converge due to relatively inaccurate surface integrals. Good Luck. T. A. Keith keith@kbw350.chem.yale.edu