From chs1nt@surrey.ac.uk Fri Aug 6 15:21:12 1993 From: Dr Nicholas P Tomkinson Message-Id: <9308061424.AA11707@central.surrey.ac.uk> Subject: gaussian benzene To: chemistry@ccl.net Date: Fri, 6 Aug 93 15:24:09 BST Hi again I would like to thank those who responded to my query about gaussian calculation of the IR spectrum of benzene. It seems that one has to scale frequencies by about 0.9 to match experiment. Dario Estrin writes > There is a very nice reference about the IR spectrum of > benzene in : > J. Chem. Phys. 98, 4793 (1993). > A density functional calculation is performed in that > paper but an extensive review about what is done > (discussing problems) using other methods is > also there. Doug Fox writes > On the topic of benzene itself, Lionel Goodman and co-workers were > embarked on a very detailed analysis of benzene using Gaussian several > years ago. I believe the reference is JCP 92, 4115-24 (1990) but this > a good place to start. Nico van Eikema Hommes writes > A very good reference is "Exploring Chemistry with Electronic Structure > Methods", a book available from Gaussian Inc. (costs $35 or so). On page > 78, the necessity of scaling to correct for neglect of electron correlation > and the use of the harmonic approximation in the calculation of the > frequencies is discussed. The usual scaling factor of 0.89 was derived for > the HF/6-31G* level; for MP2/6-31G* a value of 0.93 is recommended. and John Erikson and others suggested > See "Ab Initio Molecular Orbital Theory" by Hehre, Radom, Schleyer and Pople. Thanks again Nick Tomkinson From dandy@stokes.LANCE.ColoState.Edu Fri Aug 6 02:43:50 1993 Message-Id: <199308061445.AA04284@oscsunb.ccl.net> From: David S. Dandy Subject: MMP2 and > 100 atoms To: CHEMISTRY@ccl.net Date: Fri, 6 Aug 93 8:43:50 MDT I need to use MMP2 on fairly large molecules (300 < atoms < 400), but unfortunately, my source code is hard-wired for 100 atoms max. I've gone through the exercise of replacing all dimensions with with larger values (using parameters, a la MM3(92)), but that, I'm learning is not all there is. For example, part of the data structure makes it impossible to have more than 128 atoms, regardless of the dimensions of the arrays. The discovery that there are intrinsic limitations on structure size built into the code have lowered my confidence that all of these subtleties will be identified and addressed. Has anyone modified MM2 or MMP2 to allow for more than 100 atoms? Any guidance that might be offered would be greatly appreciated. --David Dandy -- ======-----=====-----=====-----=====-----=====-----=====-----====== David S. Dandy Dept. of Chemical Engineering (303) 491-7437 Colorado State University dandy@stokes.LANCE.ColoState.Edu Fort Collins, Colorado 80523 From Y0XIAO01@ULKYVX.LOUISVILLE.EDU Fri Aug 6 06:11:44 1993 Date: Fri, 06 Aug 1993 11:11:44 -0500 (EST) From: "Leon Y. Xiao, (502)635-6931" Subject: Re: MD description To: FENG@LISBOA.KS.UIUS.EDU, CHEMISTRY@ccl.net Message-Id: <01H1F8M4JOS28WX3H1@ULKYVX.LOUISVILLE.EDU> Hi, Feng: In your description: >2. Where to get a copy of MD >A copy of MD may be obtained by anonymous ftp from lisboa.ks.uiuc.edu, >in the directory /pub/md. The versions currently available are >md2.0.tar.Z (which includes sample data files) or md2.0-nd.tar.Z >(which contains no data files). After ftp'ing the version you prefer, >do the following: > > mkdir md; > cd md; > zat ../md2.0.tar.Z | tar xvf - ^^^^^ I think the last line should be: zcat ../md2.0.tar.Z | tar xvf - -------------------------------- Y.L.Xiao Dept. Chem., U of L From elewars@alchemy.chem.utoronto.ca Fri Aug 6 17:09:43 1993 Date: Fri, 6 Aug 93 21:09:43 -0400 From: elewars@alchemy.chem.utoronto.ca (E. Lewars) Message-Id: <9308070109.AA00907@alchemy.chem.utoronto.ca> To: chemistry@ccl.net Subject: Energies at HF and MP2 levels Dear netters: I have been studying the potential energy surface for the neutrals and radical cations of CH2Cl2 and its isomers. My problem is this: for the radical cations (charge=1, multiplicity=2) CH2Cl2.+ (1) and H2C-Cl-Cl.+ (2), both C2v, the energies I found are: UHF/6-31G* (1) -957.567 059 (2) -957.600 186 UMP2(FC)/6-31G* (1) -957.970 817 (2) -957.950 117 So at the uncorrelated level, (2) is 87 kJ/mol below (1), but at the MP2(FC) level, (1) is 54 kJ/mol below (2). ZPE corrections are small, and MP4(SDTQ) single-pt. calculations at the MP2 geometries give relative energies (incl. ZPE's) of 0/37 kJ for (1)/(2). It seems strange that at the HF level (2) should be considerably stabler than (1), while at the MP2 level the reverse is true. I think the species are in their ground electronic states, as slight distortion to C1 symmetry makes no difference to the energies (a C1 structure should not converge on an excited state). DOES IT LOOK LIKE SOMETHING IS WRONG HERE, OR IS THIS KIND OF ENERGY INVERSION ON GOING TO THE MP2 LEVEL NOT SO ODD? It may be relevant that 1. The geometries do not change much on going to the MP2 level. 2. (2) has (MP2) C-Cl 1.589, Cl-Cl 3.407, so the chlorines are only weakly bonded together (a van der Waals molecule?) 3. For the neutrals corresponding to (1) and (2) (charge=o, mult.=1), the tetrahedral CH2Cl2 is lower in energy than H2C-Cl-Cl (Cs) at both the HF and MP2(FC) levels, in our hands and as reported by Maier et al (JACS 1990 112, 5117). For neutral H2C-Cl-Cl, MP2(FC)/ 6-31g*, C-Cl is 1.630 and Cl-Cl 2.416 (cf. Cl2, Cl-Cl 2.015). Of course all these species are relative minima at the two levels (no imaginary frquencies). Thanks for any help. Errol =============================================================================== elewars@trentu.ca E. Lewars FAX (705)748-1625 Chemistry Dept Trent University Peterborough, Ontario Canada K9J 7B8 ===============================================================================