An accurate initial guess can be generated for molecular systems by superimposing converged molecular orbitals on isolated fragments. This initial guess is requested by specifying FRAGMO option for SCF_GUESS keyword and can be used for both the conventional SCF methods and the locally-projected SCF methods. The number of SCF iterations can be greatly reduced when FRAGMO is used instead of SAD. This can lead to significant time savings for jobs on multifragment systems with large basis sets [747]. Unlike the SAD guess, the FRAGMO guess is idempotent.
To converge molecular orbitals on isolated fragments, a child Q-Chem job is executed for each fragment. $rem variables of the child jobs are inherited from the $rem section of the parent job. If SCF_PRINT_FRGM is set to TRUE the output of the child jobs is redirected to the output file of the parent job. Otherwise, the output is suppressed.
Additional keywords that control child Q-Chem processes can be set in the $rem_frgm section of the parent input file. This section has the same structure as the $rem section. Options in the $rem_frgm section override options of the parent job. $rem_frgm is intended to specify keywords that control the SCF routine on isolated fragments. Please be careful with the keywords in $rem_frgm section. $rem variables FRGM_METHOD, FRGM_LPCORR, JOBTYPE, BASIS, PURECART, ECP are not allowed in $rem_frgm and will be ignored. $rem variables FRGM_METHOD, FRGM_LPCORR, JOBTYPE, SCF_GUESS, MEM_TOTAL, MEM_STATIC are not inherited from the parent job.
Example 12.272 FRAGMO guess can be used with the conventional SCF calculations. $rem_frgm keywords in this example specify that the SCF on isolated fragments does not have to be converged tightly.
$molecule
0 1
--
0 1
O -0.106357 0.087598 0.127176
H 0.851108 0.072355 0.136719
H -0.337031 1.005310 0.106947
--
0 1
O 2.701100 -0.077292 -0.273980
H 3.278147 -0.563291 0.297560
H 2.693451 -0.568936 -1.095771
--
0 1
O 2.271787 -1.668771 -2.587410
H 1.328156 -1.800266 -2.490761
H 2.384794 -1.339543 -3.467573
--
0 1
O -0.518887 -1.685783 -2.053795
H -0.969013 -2.442055 -1.705471
H -0.524180 -1.044938 -1.342263
$end
$rem
JOBTYPE SP
METHOD EDF1
BASIS 6-31(2+,2+)g(df,pd)
SCF_GUESS FRAGMO
SCF_PRINT_FRGM FALSE
$end
$rem_frgm
SCF_CONVERGENCE 2
THRESH 5
$end