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 multi-fragment systems with large basis sets.Khaliullin:2007 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, and SCF_GUESS are not inherited from the parent job.
The use of FRAGMO guess is also supported when GEN_SCFMAN = TRUE. It is extended to support more SCF orbital types (R/U/RO/G). Meanwhile, users are allowed to read in the previously generated FRAGMO guess instead of recalculating them if there is no difference between these jobs on the fragment level. This can be particularly useful for scenarios such as scanning a potential energy curve for an intermolecular complex, or for restarting an EDA job. This is controlled by the $rem variable FRAGMO_GUESS_MODE.
$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 METHOD EDF1 BASIS 6-31(2+,2+)g(df,pd) SCF_GUESS FRAGMO SCF_PRINT_FRGM FALSE $end $rem_frgm SCF_CONVERGENCE 2 $end
$molecule 1 2 -- 0 2 C1 H1 C1 1.09267 H2 C1 1.09267 H1 107.60335 H3 C1 1.09267 H2 107.60335 H1 115.692 -- 1 1 Na C1 scan H3 111.28008 H2 -122.154 scan = 2.0 $end $rem METHOD b3lyp BASIS 6-31g GEN_SCFMAN true UNRESTRICTED false SCF_CONVERGENCE 8 ROSCF true SCF_GUESS fragmo THRESH 14 SYMMETRY false SYM_IGNORE true $end @@@ $molecule 1 2 -- 0 2 C1 H1 C1 1.09267 H2 C1 1.09267 H1 107.60335 H3 C1 1.09267 H2 107.60335 H1 115.692 -- 1 1 Na C1 scan H3 111.28008 H2 -122.154 scan = 3.0 $end $rem METHOD b3lyp BASIS 6-31g GEN_SCFMAN true UNRESTRICTED false SCF_CONVERGENCE 8 ROSCF true SCF_GUESS fragmo FRAGMO_GUESS_MODE 2 !read in the available fragment data THRESH 14 SYMMETRY false SYM_IGNORE true $end