Geometry Optimizations Lecture CompChem 2 Chemistry 347 Hope College.

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Presentation transcript:

Geometry Optimizations Lecture CompChem 2 Chemistry 347 Hope College

Potential Energy Surfaces Reaction Coordinate Energy 3N-5 Coords

PES Extrema (Stationary Points) Minima –1st deriv (gradient) equals zero in all directions –2nd deriv positive in all directions –Global vs. local Saddle Point –“Transition state” –2nd deriv negative in one direction, positive in others Hilltop –2nd deriv negative in more than one direction

Locating PES Extrema Algorithms –Minimize energy (local minima) –Minimize gradient (transition state) –Specialized methods Convergence criteria –Force (  E/  r)  0 –Displacement (  r)  0 Indicated in output file

Geometry Optimization Quick Start –WebMO –Build fluoroethene, C 2 H 3 F –Gaussian –Geometry Optimization, Hartree-Fock, 3-21G, Preview Input File –Note bond angles; Submit Job –View; Note bond angles –Raw Output

General Strategies A good starting geometry is essential For large molecules, pre-optimize using a lower level of theory When a job fails –Read the output file to determine why –Look at the final geometry (Restart from Job Manager) Although every molecule is different, past experience will help guide your calculations

Locating Transition States Very difficult! Finding 1 point in 10 3N-6 Opt=(TS,CalcFC,NoEigenTest) –Minimize gradient –Must have a good starting point Opt=QST2 –“Morph” reactants and products –Must provide both structures –Atoms must have same numbering!

Advanced Features Opt=CalcFC or Opt=CalcAll –Better computation of force constants Opt=(MaxCyc=n) –Increase maximum number of iterations Use checkpoint files (built into WebMO) –%Chk=output.chk to produce a checkpoint file –Geom=(Check,Step=n) to read checkpoint file Multiple options can be specified by Opt=(TS,CalcFC,MaxCyc=100)