Setting up a Level of Theory

Once you have drawn your molecule with the geometry you want, you need to set up the type of computation (or ``level of theory'') for the Q-Chem module. First make sure that the title HyperChem Menus appears to the left of Help in the menu bar. If you see QChem Menus, then click it so that the title changes to HyperChem Menus. This will give you access to the Q-Chem menus in the menu bar. Now go to the Setup menu and select Choose Basis Set. Click the button next to the basis set of your choice, or click Other to get additional choices. Then you need to select Apply Basis Set to place your basis set on all the atoms in your molecule. When you apply the basis, make sure either no atoms or all atoms are selected. Otherwise, the basis will only be applied to the selected atoms. You should not change anything under Options or Advanced Options.

Once the basis is applied, select Q-Chem Exchange and Correlation under the Setup menu to set up the correlation treatment. For Hartree-Fock and MP2 computations, Exchange should be set to Hartree-Fock. For DFT computations, you may select whichever Exchange setting you like, except for Hartree-Fock exchange, which doesn't really make sense for DFT. The value of the Correlation selection should be No correlation for your Hartree-Fock computations (since HF doesn't include correlation by definition). For MP2, select MP2, and for DFT select whichever correlation option you like, except for No correlation, MP2, or CIS(D). Note that for DFT methods, certain choices for Exchange will override the choice of Correlation. If you pick B3LYP or B3PW91, this automatically selects your correlation treatment for you (LYP or PW91, respectively). Additionally, if you choose EDF1 exchange, you ought to select LYP(EDF1) correlation.