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Doing A Energy-only Geometry Optimization In Parallel

While geometry optimizations are best done with analytic gradients (and CFOUR has analytic gradients for most methods), an energy-only geometry optimization is something which one should avoid doing. However, there are cases (for example, EOM-CCSDT) where there are no gradients available for a useful method, and the METHOD=ENERONLY approach to a geometry optimization is unavoidable. Running such a calculation in serial can be quite costly as a number of energy points must be run to evaluate each numerical gradient used in the optimization. Fortunately, a strategy entirely analogous to those used for parallel harmonic and anharmonic frequencies can also be used for optimizations, which is outlined here.

First, construct your ZMAT file for the optimization, making sure to use the keywords METHOD=ENERONLY and FREQ_ALGORITHM=PARALLEL (actually you are not calculating frequencies, so this is perhaps a strange keyword choice...). An example is given below for formamide, using SCF and a small basis (which makes this a good guinea-pig test case to make sure you are doing all steps correctly).

formamide
O
C 1 RCO*
H 2 RCH* 1 A1*
N 2 RCN* 1 A2* 3 T180
H 4 NH1* 2 A3* 1 T0
H 4 NH2* 2 A4* 1 T180

RCO = 1.4035727376
RCH = 1.0822141466
A1 = 119.7453132747
RCN = 1.3967624803
A2 = 115.5916277063
NH1 = 1.0043425811
A3 = 120.6830303327
NH2 = 1.0035589108
A4 = 120.0229720517
T180 = 180.000000000000000
T0 = 0.000000000000000

*CRAPS(CALC=SCF,ICONTL=8
BASIS=DZ,SCF_CONV=11
METHOD=ENERONLY
FREQ_ALGORITHM=PARALLEL)
Now, run the pfindif_setup script which is described in the section