Applications I: Partial Charges and Potential Energy Surface Scans

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Applications I Partial Charges and Potential
Energy Surface Scans

• Lecture CompChem 5Chemistry 347Hope College

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Previous Applications

• Geometry optimization
• Conformation stability
• Transition states
• IR and NMR spectra
• Thermochemistry (ZPE, CV, S, H, G)
• Enthalpies of reaction DrxnH

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Partial Charges

• No unique definition of assigning electrons to
  atoms
• Mulliken populations derived from AO
  coefficients and overlap integrals
• Natural Bonding Orbitals derived by assigning
  electrons to core, valence, and lone pair
  orbitals
• Electrostatic potential-derived charges derived
  to match electrostatic potential at a given
  distance

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Rigid Potential Energy Surface Scan

• SCAN keyword
• Series of Single Point calculations that
  systematically vary one (or more) coordinate
• Non-stepped coordinates are fixed, not optimized
• Relatively fast, but of limited utility

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Relaxed Potential Energy Surface Scan

• OptZ-matrix or OptAddRedundant keyword
• A series of Optimize Geometry calculations that
  systematically vary one (or more) coordinate
• Stepped coordinate must be carefully chosen to
  represent chemical feature of interest
• Available in Z-matrix (easier input file) or
  redundant coordinates (more robust method)