Quantum Mechanics Calculations III Apr 2010 Postgrad course on Comp Chem Noel M. O’Boyle

Overview of QM methods Including correlation HF (“ab initio”) Semi-empirical Forcefields DFT Speed/Accuracy Molecular mechanics Quantum mechanics (wavefunction) Quantum mechanics (electron density)

Carrying out calculations ware Gaussian (commercial) –The most widely used package –Can do most everything, and is pretty robust. Presumably not the fastest (the only software whose license prohibits published comparisons). –Gaussian09 available through ICHEC, also faster (8-CPU machines) ORCA (free for academics) –General purpose – Lots of other commercial packages: ADF (for DFT), Jaguar MOPAC2009 (commercial but free for academics) –Semi-empirical calculations, Stewart NWChem (free for academics) –General purpose – GAMESS (available for free) –General purpose calculations. (See next page) Firefly –Fork of GAMESS, formerly “PC GAMESS” –Faster than GAMESS (especially on Windows) PSI3 (Open Source) –HF and higher level – no Windows support

Once you have the QM software... Set up calculations –Avogadro (OS, –GaussView (commercial, available when buying Gaussian) Analyse results –Avogadro –Molekel (OS, –GaussSum (OS, by me) –MacMolPlt (free, –...and many others

Get GAMESS GAMESS is free (but not open-source) software developed by the Gordon Group and collaborators To get it... –Go to –Click on “How to get GAMESS” –Click “Obtaining GAMESS” –Click “I agree” –Enter address, and tick box for “GAMESS version January 12, 2009 R3 for Microsoft Windows” (also a ChemOffice version – haven’t tried it) –Click “Submit request” –You will receive an with a link, and a username and password –Follow the link to download the installer and enter the username and password when requested To install it... –Run the installer –Click Next a few times –It installs to C:\WinGAMESS To use it... –Documentation is in C:\WinGAMESS\manuals –Following C:\WinGAMESS\manuals\WinGAMESS.pdf we can... Create a shortcut to C:\WinGAMESS\WG_DDE.bat on the desktop Drag-and-drop GAMESS input files onto this shortcut How do we create an input file? –Install Avogadro and MacMolPlt

Input for a calculation What type of calculation to do Location and atomic number of atoms Charge on molecule Multiplicity of molecule No need for bond orders No need to say where the charge is –The charge is simply used to calculate the total number of electrons Typical calculations are in vacuo –i.e. not in solid or liquid state, or in solvent –If required, there is software for handling bulk solids, and there are methods to include solvent effects

Jan Jensen’s GAMESS Screencasts Look at all of Jan’s screencasts and posts starting from the oldest –On the right-hand side, see under Blog Archive Tools of the trade (all free, some open source) – Building a molecule – 3d.htmlhttp://molecularmodelingbasics.blogspot.com/2009/07/building-complicated-molecule-2d-to- 3d.html Finding the lowest energy conformer – and.html Look at the GAMESS input file – Set up a calculation using Avogadro – Finding a transition state – reaction.htmlhttp://molecularmodelingbasics.blogspot.com/2009/08/finding-transition-state-sn2- reaction.html Finding the path connecting reactant to product (intrinsic reaction coordinate) –

Practical tips To speed up a geometry optimization, you can optimise first with a smaller basis set and then continue from there If you have trouble with SCF convergence, it can be a good idea to try converging it with a smaller basis set first and then projecting from that as the initial guess Molecules with geometrical symmetry will be corresponding faster (on the other hand, if you require symmetry to be broken during the calculation, you should turn off symmetry perception)