Geometry optimizations are invoked by the optg input directive, which must follow the input of the method for which the optimization is to be performed. For instance, optimization of the geometry for formaldehyde at the MP2 level is performed with the following input
***,formaldehyde print,basis,orbitals !this is optional: print the basis set and the !occupied orbitals geometry !define the nuclear coordinates ang C O , C , rco H1 , C , rch , O , hco H2 , C , rch , O , hco , H1 , 180 end rco=1.182 Ang rch=1.102 Ang hco=122.1789 Degree basis=vdz !Select basis set hf !Perform HF calculation mp2 !Perform MP2 calculation optg !Optimize geometry for MP2
Optimizations use analytical energy gradients if available; otherwise the gradients are computed by finite differences. Presently, analytical energy gradients are available for the following methods.
Various options are available for modifying the convergence thresholds and the optimization method. For details see the reference manual.