Carlier Group Gaussian User Manual - Virginia Tech

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22 Carlier Group Gaussian User Manual See Sections 07. and 08. below on Monitoring Calculations and Visualizing Gaussian Output to determine the number of imaginary frequencies and to see the molecular motion corresponding to the imaginary frequency. 06. Constrained Optimizations A. Setup To perform constrained optimizations, the key command is #rb3lyp/6-31g(d) opt=modredundant You can use internal or cartesian coordinates. As usual, I prefer cartesian coordinates. The key is to know the atom numbers for your constraint. You can get these by looking at the structure in Molden (GaussView should also work). One atom constrains an atom to not move in 3D space. Two atoms define a distance, three an angle, and four contiguous atoms define a dihedral angle. Four non-contiguous atoms constitute an improper torsion which is a useful way to define or control the orientation of a group (D) relative to a plane (ACB). A B D A B D A B D θ C C C the relationship of D to the plane defined by ABC can be measured in terms of improper torsion or deformation angle Note that the improper torsion angle has the benefit of being signed, whereas a more conventional deformation angle is not signed. After the geometry specification, include a carriage return, and then specify the distance, angle or dihedral you want to constrain, using the appropriate atom numbers; following this you can several different things Examples of dihedral angle/improper torsion constraints 27 25 12 11 F this would fix the 27 25 12 11 dihedral to what it is in the initial geometry: F stands for fixed. 27 25 12 11 10.0 F this would fix the 27 25 12 11 dihedral to 10.0 degrees (note floating point, dihedral is different from initial geometry)
Carlier Group Gaussian User Manual 23 27 25 12 11 S 2 10.0 this would perform a series of constrained optimizations, where the 27 25 12 11 dihedral is first constrained to its original value, and then incremented twice by 10.0 degrees. Here S stands for ‘step.’ Note the use of the floating point number for the increment. 27 25 12 11 30.0 S 8 10.0 this would perform a series of constrained optimizations, where the 27 25 12 11 dihedral is first constrained to 30 degrees (different from its original value), and then incremented eight times by 10.0 degrees. Note the use of the floating point number for the increment. B. Multiple Constraints I have had occasion to perform optimizations with one fixed dihedral constraint and one dynamic dihedral constraint. As far as I can remember these have all failed. No atoms were shared in the constraints, and these calculations were also problematic in Spartan. What I have been able to do is perform optimizations with two fixed dihedral constraints C. Monitoring constrained optimizations To check conveniently--find out the internal coordinate name for your constraint--this will be R# for a bond length, A# for an angle, D# for a dihedral egrep D37 h0053.out this will show you the final value for your constraint when you have a series of calculations in the same job, and want to extract the energies, vim filename.out :/Summary this will take you to a list of eigenvalues left to right, under columns representing each optimized step in the calculation. The eigenvalue is the energy. Note that semiempirical energies are given in hartrees and are very small (since they reflect heat of formation, not electronic energy). If you want to get the geometries and energies out vim filename.out :/Optimized then scroll up to SCF done--your energy for the first optimization is here.
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Carlier Group Gaussian User Manual - Virginia Tech

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