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DFT calculations with ADF Density Functional Theory • A detailed description of DFT will not be included in this presentation. • Notes which help to explain what keywords correspond to will be included, but in a very loose, unrigourous way. • For further details about DFT, the following books are recommended. I also recommend you attend an appropriate lecture series – A chemists guide to DFT, W. Koch, M. C. Holthausen, Wiley- VCH, ISBN: 3-527-30372-3 – Computational Chemistry, G.H. Grant, W. G. Richards, OUP (Oxford Primer), ISBN: 978-0198557401 – Introduction to computational chemistry, F. Jensen, Wiley, ISBN: 0-470-01187-4 Amsterdam Density Functional • ADF is a program used to do DFT calculations • www.scm.com is the website for the program and includes – Manual (under the documentation heading) – Forum where people post problems and others reply (under community) • Also look on ccl.net, although this is more general and ADF issues tend to get directed to the ADF website. Example input file $ADFBIN/adf -n 1 <<eor TITLE H2O BASIS TYPE TZP CORE Small END SCF ITERATIONS 99 CONVERGE 0.000001 END GEOMETRY optim END INTEGRATION 6.0 XC GRADIENT OPBE END CHARGE 0 0 ATOMS O 0.000000 0.000000 0.012976 H 0.000000 -0.754442 -0.589982 H 0.000000 0.754442 -0.589982 END ENDINPUT eor Example input file - explained $ADFBIN/adf -n 1 <<eor This line needs to be included in all ADF calculations TITLE H2O Folder where Use ADF executable Use one processor BASIS ADF binaries (there are others for TYPE TZP (irrelevant for machines CORE Small are kept different programs END with only 1 cpu) and utilities) SCF ITERATIONS 99 CONVERGE 0.000001 END Read this file until GEOMETRY you reach “eor” optim END INTEGRATION 6.0 XC GRADIENT OPBE END CHARGE 0 0 ATOMS O 0.000000 0.000000 0.012976 H 0.000000 -0.754442 -0.589982 H 0.000000 0.754442 -0.589982 END ENDINPUT Specifies the input file is finished eor Stop reading the file (see first line) Example input file - explained $ADFBIN/adf -n 1 <<eor TITLE H2O You can give your calculation whatever title you like or exclude this keyword altogether BASIS TYPE TZP CORE Small BASIS keyword specifies what type of basis set you want END to use. SCF TYPE can be lower case but TZP (or whatever you ITERATIONS 99 choose) must be capitals as it points to a directory CONVERGE 0.000001 CORE specifies how large your frozen core is, case END specific GEOMETRY optim END specifies that this is the end of this section END SCF specifies the conditions for putting the electron INTEGRATION 6.0 density correctly XC GRADIENT OPBE Iterations says how many cycles it has to converge in END Converge says how much the energy is allowed to change CHARGE 0 0 by to be considered converged ATOMS O 0.000000 0.000000 0.012976 H 0.000000 -0.754442 -0.589982 H 0.000000 0.754442 -0.589982 END ENDINPUT eor Some definitions • BASIS SET – These are mathematical functions which describe where the electron density is allowed to go. The proportion of each function can be varied until the energy provided by the electron density is minimised. In ADF the basis functions are the shape of s, p, d etc orbitals • FROZEN CORE – The innermost electrons of an element contribute a great deal to the overall energy but are largely unaffected by the overall environment of the atom. Therefore they can be optimised once and then kept the same throughout the calculation as the valence electrons are the ones which change most. Some definitions • Self Consistent Field (SCF) – This is how the electron density is distributed within the field of the nuclei and other electrons. An iterative procedure is used to assign the electron density until it is internally consistent, and changing the position of one electron a tiny bit does not change the energy overall. Example input file - explained $ADFBIN/adf -n 1 <<eor TITLE H2O BASIS Geometry keyword specifies whether you want to optimise TYPE TZP CORE Small the geometry, do a single point, or do frequencies. It can END also specify convergence criteria. SCF ITERATIONS 99 CONVERGE 0.000001 Integration relates to the accuracy of the calculation. END GEOMETRY The XC keyword is used to select the functional to be used optim END INTEGRATION 6.0 The charge keyword specifies the charge of the molecule and the number of unpaired electrons. If there are unpaired XC GRADIENT OPBE electrons another keyword, UNRESTRICTED, needs to be END included on another line CHARGE 0 0 ATOMS O 0.000000 0.000000 0.012976 H 0.000000 -0.754442 -0.589982 H 0.000000 0.754442 -0.589982 END ENDINPUT eor Some definitions • Functional – A functional is a function that acts on the basis functions to return the energy. It can be thought of that in the Schrödinger equation the Hamiltonian acts on the wavefunction to give the energy multiplied by the wavefunction. In DFT, the functional acts on the electron density (in the form of basis sets) to give the energy multiplied by the electron density. However, the exact form the functional should take is unknown so many different functionals have been created which all give slightly different results. Some definitions • RESTRICTED/UNRESTRICTED – In a calculation where there are no unpaired electrons, the spin-up and spin-down electrons are energetically and spatially identically. – In a calculation where there are unpaired electrons (multiplicity > 1), the spin-up and spin-down electrons need to be able to have different energetic and spatial properties, and the UNRESTRICTED keyword allows this. Example input file - explained $ADFBIN/adf -n 1 <<eor TITLE H2O BASIS TYPE TZP CORE Small END SCF ITERATIONS 99 CONVERGE 0.000001 Atoms keyword is where the geometry of the END molecule (location of nuclei) is included. GEOMETRY Can be xyz file or z-matrix (in which case the optim END word internal or zmat needs to be included INTEGRATION 6.0 after word atoms) XC GRADIENT OPBE END CHARGE 0 0 ATOMS O 0.000000 0.000000 0.012976 H 0.000000 -0.754442 -0.589982 H 0.000000 0.754442 -0.589982 END ENDINPUT eor Example output file • Output files are rather large so here is a summary of what is in an output file • Energy Summary of Bonding Energy (energy terms are taken from the energy decomposition above) ====================================================================================== Electrostatic Energy: -0.256031610040912 -6.9670 -160.66 -672.21 Kinetic Energy: 0.447024581673579 12.1642 280.51 1173.66 Coulomb (Steric+OrbInt) Energy: -0.247468564515559 -6.7340 -155.29 -649.73 XC Energy: -0.468398583755849 -12.7458 -293.92 -1229.78 -------------------- ----------- ---------- ----------- Total Bonding Energy: -0.524874176638742 -14.2826 -329.36 -1378.06 Hartree Electron Volt Kcal mol-1 kJ mol-1 If you want to see which structures are more stable then it is the energies you want to compare. You can only compare energies from the same method (functional, basis set, level of integration) and number of atoms/atom types. ADF calculates the energy of bringing different atoms together, i.e. the bonding energy. Looking at results • Chemcraft (www.chemcraftprog.com) – Still need to double check that this can read ADF2008 output correctly (It does read it but I am not sure if it reads the last geometry) • Can view iccg wiki too for some general details (www.warwick.ac.uk/go/iccg) • Open up file.output using chemcraft • Last structure in optimisation displayed • Can also display frequencies