From gadre@chem.unipune.ernet.in Fri Apr 4 06:33:54 1997 Received: from sangam.ncst.ernet.in for gadre@chem.unipune.ernet.in by www.ccl.net (8.8.3/950822.1) id GAA08482; Fri, 4 Apr 1997 06:17:41 -0500 (EST) Received: from iucaa (iucaa.ernet.in [144.16.31.4]) by sangam.ncst.ernet.in (8.7.5) with SMTP id QAA22208 for ; Fri, 4 Apr 1997 16:52:11 +0530 (GMT+05:30) Received: from unipune.ernet.in by iucaa (5.65v3.2/SMI-4.1) id AA32542; Fri, 4 Apr 1997 16:43:37 +0500 Received: from chem.unipune.ernet.in by unipune.ernet.in (8.8.5/UNIPUNE-HUB-2.4.97) id QAA04717; Fri, 4 Apr 1997 16:44:01 -0500 (GMT) Received: by chem.unipune.ernet.in (8.7.5/SMI-SVR4) id QAA07597; Fri, 4 Apr 1997 16:55:55 -0500 Date: Fri, 4 Apr 1997 16:55:55 -0500 From: gadre@chem.unipune.ernet.in (Prof. Shridhar R. Gadre) Message-Id: <199704042155.QAA07597@chem.unipune.ernet.in> To: CHEMISTRY@www.ccl.net Subject: XII ICCCRE XII International Conference on Computers in Chemical Research and Eduction First Annoucement The Twelfth International Conference on Computers in Chemical Research and Education (ICCCRE XII) will be held at the University of Pune in Pune, India, in the week of 5-10 January 1998. Pune is one of the premier centers of learning in India, located about 100 miles south of Mumbai (Bombay). The purpose of the ICCCRE is to present a sweeping and state-of-the-art description and critical examination of the applications of computers in chemistry presented by outstanding computational chemists. The program will be organized such that all interested parties can experience a broad range of topics. There will be ample opportunities for small group and one-on-one discussions. In particular, it is our endeavour that the program will especially cater to the needs of junior faculty, researchers, and students who are just starting out in the field. In addition, those based in other disciplines, whose work overlaps with computers applied in chemistry, will be exposed to authoritative treatments of computers in chemistry and related disciplines. Tentatively, the technical program will focus on the following topics : (1) Computers in Chemical Education (2) Electronic Journals, Publishing and Conferencing (3) Artificial Intelligence, Chemometrics, Neural Networks and their Chemical Application. (4) Molecular Modeling, Quantitative Structure-Activity Relationship (QSAR). (5) Semi-Empirical and Ab initio Quantum Chemistry, Density Functional Methods. (6) New Algorithms and Techniques in Computational Chemistry Under consideration is the possibility of pre-conference sharing of invited/contributed papers via electronic conferencing (a new feature of the ICCCRE). Given the rich and varied history and cultural attractions of India, the organizers hope to arrange pre-and/or post-conference excursions of varying duration. Programmatic suggestions and nominations for speakers are encouraged and are indeed very welcome. Please send your comments and/or let us know whether you would like to have your name and address added to the distribution list for further information via one of the following channels : (1) Official Mailing Address : ICCCRE XII c/o Professor Shridhar R. Gadre Department of Chemistry, University of Pune Pune - 411 007. INDIA e-mail : gadre@unipune.ernet.in Phone and Fax : 91-212-351728 (2) Facilitators working with Professor S.R. Gadre : Professor Peter Lykos Professor Rama Viswanathan Illinois Inst. of Technology Beloit College Chicago IL 60616. U.S.A. Beloit, WI 53511. U.S.A. 1-312-567-3430 1-608-363-2273 lykos@charlie.cns.iit.edu ramav@beloit.edu We will appreciate your comments/suggestions, please help us to publicize ICCCRE XII by forwarding and/or posting this announcement on bulletin boards, L-servs, WWW sites. newsletters, society publications and calendars and your own distribution lists. The details regarding this conference can be found at the web page : http://www.beloit.edu/~chem/ICCCCRE.html. Response Sheet XII ICCCRE Please mail to Professor Shridhar R. Gadre Department of Chemistry University of Pune Pune - 411 007 INDIA Fax : 91-212-351728 e-mail : gadre@chem.unipune.ernet.in 1. I will be interested in attending XII ICCCRE in Pune (January 5-10, 1998). 2. I plan to present A paper Yes/No A poster Yes/No 3. I/My Company would like to sponsor a Session/Satelite Meeting. Yes/No Details : ______________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ 4. Any Other Remarks/Suggestions _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ Please write your complete name and institutional address _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ Fax : e-mail Phone : From youngd2@mallard.duc.auburn.edu Fri Apr 4 09:37:04 1997 Received: from mallard.duc.auburn.edu for youngd2@mallard.duc.auburn.edu by www.ccl.net (8.8.3/950822.1) id IAA09465; Fri, 4 Apr 1997 08:51:46 -0500 (EST) Received: from wood.mail.auburn.edu by mallard.duc.auburn.edu (SMI-8.6/SMI-SVR4) id HAA16433; Fri, 4 Apr 1997 07:51:45 -0600 Received: by wood.mail.auburn.edu (SMI-8.6/SMI-SVR4) id HAA15120; Fri, 4 Apr 1997 07:51:43 -0600 Date: Fri, 4 Apr 1997 07:51:43 -0600 From: youngd2@mail.auburn.edu Message-Id: <199704041351.HAA15120@wood.mail.auburn.edu> To: CHEMISTRY@www.ccl.net Subject: Chem Topic: Spin Contamination Hello all, I have written the following short essay for my users and am posting it here for your enjoyment and comments. Please let me know if I missed any important points. My compilation of chemical topics can be accessed via the web at URL http://www.auburn.edu/~youngd2/topics/contents.html Dave Young youngd2@mail.auburn.edu --------------------------------------------------------------------------- Spin Contamination David Young Division of University Computing 144 Parker Hall Auburn University Auburn, AL 36849 WHAT IS SPIN CONTAMINATION Introductory descriptions of Hartree-Fock calculations (usually using Rootaan's SCF method) focus on singlet systems for which all electron spins are paired. By assuming that the calculations is restricted to having two electrons per occupied orbital, the computation can be done relatively easily. This is often referred to as a restricted calculation or RHF. For systems with a multiplicity other than one, it is not possible to use the RHF method as is. Often an unrestricted SCF calculation (UHF) is performed. In an unrestricted calculation, there are two complete sets of orbitals, one for the alpha electrons and one for the beta electrons. Usually these two sets of orbitals use the same set of basis functions but different molecular orbital coefficients. The advantage of unrestricted calculations is that they can be performed very efficiently. The disadvantage is that the wave function is no longer an eigenfunction of the total spin, , thus some error may be introduced into the calculation. This error is called spin contamination. HOW DOES SPIN CONTAMINATION AFFECT RESULTS Spin contamination results in having orbitals which appear to be the desired spin state, but have a bit of some other spin state mixed in. This results in slightly lowering the computed total energy. However, this lowering is an artifact of an incorrect wave function. Since this is not a systematic error, the difference in energy between states will be adversely affected. A high spin contamination can affect the geometry and population analysis and significantly affect the spin density. As a check for the presence of spin contamination, most ab initio programs will print out the expectation value of the total spin, . If there is no spin contamination this should equal s(s+1) where s equals 1/2 times the number of unpaired electrons. One rule of thumb which was derived from experience with organic molecule calculations is that the spin contamination is negligible if the value of differs from s(s+1) by less than 10%. Although this provides a quick test, it is always advisable to double check the results against experimental evidence or more rigorous calculations. Unrestricted calculations often incorporate a spin annihilation step which removes a large percentage of the spin contamination from the wave function at some point in the calculation. This helps minimize spin contamination but does not completely prevent it. The final value of is always the best check on the amount of spin contamination present. In Gaussian, the option "iop(6/15=2)" tells the program to use the annihilated wave function to produce the population analysis. I am not aware of any programs that use the annihilated wave function to perform the geometry optimization. RESTRICTED OPEN SHELL CALCULATIONS It is possible to run spin-restricted open shell calculations (ROHF). The advantage of this is that there is no spin contamination. The disadvantage is that there is an additional cost in the form of CPU time required in order to correctly handle both singly occupied and doubly occupied orbitals and the interaction between them. As a result of the mathematical method used, ROHF calculations give good total energies and wave functions but the singly occupied orbital energies don't rigorously obey Koopman's theorem. When it has been shown that the errors introduced by spin contamination are unacceptable, restricted open shell calculations are the best way to get a reliable wave function. Within the Gaussian program, restricted open shell calculations can be performed for Hartree-Fock, density functional theory, MP2 and some semiempirical wave functions. The ROMP2 method does not yet support analytic gradients, thus the fastest way to run the calculation is as a single point energy calculation with a geometry from another method. If a geometry optimization must be done at this level of theory, a non-gradient based method such as the Fletcher-Powell optimization must be used (note that the G94 manual implies that this may not still be functional for all cases). SPIN PROJECTION METHODS Another approach is to run an unrestricted calculation then project out the spin contamination after the wave function has been obtained (PUHF, PMP2). This gives a correction to the energy, but does not improve other properties. A spin projected result does not give the energy obtained by using a restricted open shell calculation. This is because the unrestricted orbitals were optimized to describe the contaminated state rather than being optimized to describe the spin projected state. HALF-ELECTRON APPROXIMATION Semiempirical programs often use the half electron approximation for radical calculations. The half electron method is a mathematical technique for treating a singly occupied orbital in an RHF calculation. This results in a consistent total energy at the expense of having an approximate wave function and orbital energies. Since a single determinant calculation is used, there is no spin contamination. The consistent total energy makes it possible to compute singlet-triplet gaps using RHF for the singlet and the half electron calculation for the triplet. Koopman's theorem is not obeyed for half electron calculations. Also, no spin densities can be obtained. The Mulliken population analysis is usually fairly reasonable. FURTHER INFORMATION Some discussion and results are in W. J. Hehre, L. Radom, P. v.R. Schleyer, J. A. Pople "Ab Initio Molecular Orbital Theory" Wiley (1986) An article that compares unrestricted, restricted and projected results is M. W. Wong, L. Radom J. Phys. Chem. 99, 8582 (1995) Some specific examples and a discussion of the half electron method are given in T. Clark "A Handbook of Computational Chemistry" Wiley (1985) A more mathematical treatment can be found in the paper J. S. Andrews, D. Jayatilake, R. G. A. Bone, N. C. Handy, R. D. Amos Chem. Phys. Lett. 183, 423 (1991) --------------------------------------------------------------------------- From youngd2@mallard.duc.auburn.edu Fri Apr 4 09:37:19 1997 Received: from mallard.duc.auburn.edu for youngd2@mallard.duc.auburn.edu by www.ccl.net (8.8.3/950822.1) id IAA09465; Fri, 4 Apr 1997 08:51:46 -0500 (EST) Received: from wood.mail.auburn.edu by mallard.duc.auburn.edu (SMI-8.6/SMI-SVR4) id HAA16433; Fri, 4 Apr 1997 07:51:45 -0600 Received: by wood.mail.auburn.edu (SMI-8.6/SMI-SVR4) id HAA15120; Fri, 4 Apr 1997 07:51:43 -0600 Date: Fri, 4 Apr 1997 07:51:43 -0600 From: youngd2@mail.auburn.edu Message-Id: <199704041351.HAA15120@wood.mail.auburn.edu> To: CHEMISTRY@www.ccl.net Subject: Chem Topic: Spin Contamination Hello all, I have written the following short essay for my users and am posting it here for your enjoyment and comments. Please let me know if I missed any important points. My compilation of chemical topics can be accessed via the web at URL http://www.auburn.edu/~youngd2/topics/contents.html Dave Young youngd2@mail.auburn.edu --------------------------------------------------------------------------- Spin Contamination David Young Division of University Computing 144 Parker Hall Auburn University Auburn, AL 36849 WHAT IS SPIN CONTAMINATION Introductory descriptions of Hartree-Fock calculations (usually using Rootaan's SCF method) focus on singlet systems for which all electron spins are paired. By assuming that the calculations is restricted to having two electrons per occupied orbital, the computation can be done relatively easily. This is often referred to as a restricted calculation or RHF. For systems with a multiplicity other than one, it is not possible to use the RHF method as is. Often an unrestricted SCF calculation (UHF) is performed. In an unrestricted calculation, there are two complete sets of orbitals, one for the alpha electrons and one for the beta electrons. Usually these two sets of orbitals use the same set of basis functions but different molecular orbital coefficients. The advantage of unrestricted calculations is that they can be performed very efficiently. The disadvantage is that the wave function is no longer an eigenfunction of the total spin, , thus some error may be introduced into the calculation. This error is called spin contamination. HOW DOES SPIN CONTAMINATION AFFECT RESULTS Spin contamination results in having orbitals which appear to be the desired spin state, but have a bit of some other spin state mixed in. This results in slightly lowering the computed total energy. However, this lowering is an artifact of an incorrect wave function. Since this is not a systematic error, the difference in energy between states will be adversely affected. A high spin contamination can affect the geometry and population analysis and significantly affect the spin density. As a check for the presence of spin contamination, most ab initio programs will print out the expectation value of the total spin, . If there is no spin contamination this should equal s(s+1) where s equals 1/2 times the number of unpaired electrons. One rule of thumb which was derived from experience with organic molecule calculations is that the spin contamination is negligible if the value of differs from s(s+1) by less than 10%. Although this provides a quick test, it is always advisable to double check the results against experimental evidence or more rigorous calculations. Unrestricted calculations often incorporate a spin annihilation step which removes a large percentage of the spin contamination from the wave function at some point in the calculation. This helps minimize spin contamination but does not completely prevent it. The final value of is always the best check on the amount of spin contamination present. In Gaussian, the option "iop(6/15=2)" tells the program to use the annihilated wave function to produce the population analysis. I am not aware of any programs that use the annihilated wave function to perform the geometry optimization. RESTRICTED OPEN SHELL CALCULATIONS It is possible to run spin-restricted open shell calculations (ROHF). The advantage of this is that there is no spin contamination. The disadvantage is that there is an additional cost in the form of CPU time required in order to correctly handle both singly occupied and doubly occupied orbitals and the interaction between them. As a result of the mathematical method used, ROHF calculations give good total energies and wave functions but the singly occupied orbital energies don't rigorously obey Koopman's theorem. When it has been shown that the errors introduced by spin contamination are unacceptable, restricted open shell calculations are the best way to get a reliable wave function. Within the Gaussian program, restricted open shell calculations can be performed for Hartree-Fock, density functional theory, MP2 and some semiempirical wave functions. The ROMP2 method does not yet support analytic gradients, thus the fastest way to run the calculation is as a single point energy calculation with a geometry from another method. If a geometry optimization must be done at this level of theory, a non-gradient based method such as the Fletcher-Powell optimization must be used (note that the G94 manual implies that this may not still be functional for all cases). SPIN PROJECTION METHODS Another approach is to run an unrestricted calculation then project out the spin contamination after the wave function has been obtained (PUHF, PMP2). This gives a correction to the energy, but does not improve other properties. A spin projected result does not give the energy obtained by using a restricted open shell calculation. This is because the unrestricted orbitals were optimized to describe the contaminated state rather than being optimized to describe the spin projected state. HALF-ELECTRON APPROXIMATION Semiempirical programs often use the half electron approximation for radical calculations. The half electron method is a mathematical technique for treating a singly occupied orbital in an RHF calculation. This results in a consistent total energy at the expense of having an approximate wave function and orbital energies. Since a single determinant calculation is used, there is no spin contamination. The consistent total energy makes it possible to compute singlet-triplet gaps using RHF for the singlet and the half electron calculation for the triplet. Koopman's theorem is not obeyed for half electron calculations. Also, no spin densities can be obtained. The Mulliken population analysis is usually fairly reasonable. FURTHER INFORMATION Some discussion and results are in W. J. Hehre, L. Radom, P. v.R. Schleyer, J. A. Pople "Ab Initio Molecular Orbital Theory" Wiley (1986) An article that compares unrestricted, restricted and projected results is M. W. Wong, L. Radom J. Phys. Chem. 99, 8582 (1995) Some specific examples and a discussion of the half electron method are given in T. Clark "A Handbook of Computational Chemistry" Wiley (1985) A more mathematical treatment can be found in the paper J. S. Andrews, D. Jayatilake, R. G. A. Bone, N. C. Handy, R. D. Amos Chem. Phys. Lett. 183, 423 (1991) --------------------------------------------------------------------------- From ccl@www.ccl.net Fri Apr 4 11:34:03 1997 Received: from bedrock.ccl.net for ccl@www.ccl.net by www.ccl.net (8.8.3/950822.1) id LAA11233; Fri, 4 Apr 1997 11:31:29 -0500 (EST) Received: from theory.tc.cornell.edu for kshippos@tc.cornell.edu by bedrock.ccl.net (8.8.3/950822.1) id LAA10560; Fri, 4 Apr 1997 11:31:26 -0500 (EST) Received: from [128.84.51.45] (FIREFLY.TC.CORNELL.EDU [128.84.51.45]) by theory.tc.cornell.edu (8.8.4/8.8.3/CTC-1.0) with ESMTP id LAA04709; Fri, 4 Apr 1997 11:28:25 -0500 Message-Id: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Date: Fri, 4 Apr 1997 11:28:24 -0500 To: Recipient List Suppressed:;@ccl.net From: kshippos@TC.Cornell.EDU (Kathy Shippos) Subject: Virtual Workshop --Using Scientific Visualization offered by CTC Cornell Theory Center CTC Virtual Workshop Using Scientific Visualization June 2-27, 1997 Registration deadline: May 1, 1997 Cornell Theory Center (CTC) has expanded its successful Virtual Workshop (VW) series to include visualization. This four week workshop introduces concepts of scientific visualization and shows how to visualize four general categories of data. Hands on examples use IBM's Data Explorer (DX). The VW allows you to learn new concepts in high performance computing and visualization at your own pace from your own machine. Details and registration can be found at: http://www.tc.cornell.edu/Edu/VW/Viz97 Kathy Shippos Conference Assistant Cornell Theory Center Cornell University Ithaca, NY 14853 kshippos@tc.cornell.edu (607) 254-8640 From ccl@www.ccl.net Fri Apr 4 19:11:33 1997 Received: for ccl@www.ccl.net by www.ccl.net (8.8.3/950822.1) id TAA14082; Fri, 4 Apr 1997 19:11:33 -0500 (EST) Date: Fri, 4 Apr 1997 19:11:33 -0500 (EST) From: Computational Chemistry Message-Id: <199704050011.TAA14082@www.ccl.net> To: boden@mol.f.u-tokyo.ac.jp, chemistry@www.ccl.net Subject: Comp-Chem-List You, i.e.: chemistry are now unsubscribed from Computational Chemistry List. Do not be alarmed if you get few more messages from the list after this note. It would only mean that some messages were in the mail distribution queue before I deleted your name. Thank you for participating. You can always resubscribe by sending a short message to chemistry-request@www.ccl.net. Jan Labanowski (or one of my coworkers) Ohio Supercomputer Center 1224 Kinnear Rd Columbus, OH 43212-1163 ph. 614-292-9279, FAX: 614-292-7168 E-mail: jkl@ccl.net or JKL@OHSTPY.BITNET