The Q-Chem Interface

Introduction

Command Line Arguments

Notes and references

1. Using the argument "basis" the basis set needs to be specified in Q-Chem nomenclature. In particular note that the basis set Ahlrichs SVP is denoted as VDZ** in Q-Chem.
2. If a basis set is taken from the ChemShell library the basis set is explicitly included in the Q-Chem input within a $basis-section and the Q-Chem $rem-variable basis is set to "gen" (see Q-Chem manual). As for an explicit basis set the default guess (Superposition of atomic densities, SAD) is not available, the guess is created by basis set projection based on an initial STO-3G calculation.
3. The different levels determine the SCF accuracy concerning three aspects: (A) The SCF convergence, (B) the threshold for density based Schwarz screening of electron repulsion integrals and (C) the DFT grid size.

   keyword	(A)	(B)	(C)
   debug	10-3	10-6	"Low quality" grid
   lowest	10-4	10-7	SG-0
   low	10-5	10-8	SG-0
   medium	10-6	10-9	SG-1
   high	10-7	10-10	75/302
   highest	10-8	10-11	110/590

   Use the largest affordable accuracy. Typically "medium" is sufficient for standard SCF energetics but you might need to choose "high" for RI-MP2, gradients or also for large QM regions! For details concerning the grid abbreviations see Q-Chem manual.
4. If not specified the DFT grid is chosen according to the argument "accuracy".
5. The Q-Chem scratch is written to $QCSCRATCH/jobname.scr/
6. B3LYP and B3LYP5 differ in the correlation functional:

   B3LYP	(correlation: 0.8100 LYP + 0.1900 VWN1RPA)
   B3LYP5	(correlation: 0.8100 LYP + 0.1900 VWN )

   For example, Q-Chem-B3LYP corresponds to Turbomole-B3LYP_GAUSSIAN (and Gaussian-B3LYP), while Q-CHEM-B3LYP5 corresponds to Turbomole-B3LYP (and original parameters). Possible remaining differences are due to different DFT grids (see argument "accuracy" or "grid").
7. Use this Hamiltonian for RI-MP2 calculations with SVP auxiliary basis set. In addition specify the "basis" argument as usual [1].
8. Make sure you physically provide the specified RAM. For large QM regions the default (2GB) needs to be increased. See Q-Chem manual for details.
9. Make sure you physically provide the specified RAM. For large QM regions the default (100MB) needs to be increased. See Q-Chem manual for details.
10. If the "purecart" argument is used, the specifications by "harmonic" are overwritten. See Q-Chem manual for the purecart variable.
11. Q-Chem provides the core hamiltonian guess (CORE), the superposition of atomic densities (SAD, default), and the generalized Wolfsberg-Helmholtz guess (GWH). Additionally by the keyword "READ" an adequate guess can be read in from file. For this purpose, the file containing the guess needs to be located in the scratch directory, i.e. $QCSCRATCH/jobname.scr/. See Q-Chem manual for details.
12. Switch on for large QM systems as "separate_jk" is required to make use of CFMM [13] and LinK [15].
13. C. A. White, B. G. Johnson, P. M.W. Gill and M. Head-Gordon, The Continuous Fast Multipole Method, Chem. Phys. Letters, 230, 8 (1994).
14. Use the default (-1) unless you really want to change the CFMM behavior (-1: Program decides best value turning on CFMM when useful, 1: Do not use CFMM, n >=8: Use CFMM with n lowest-level boxes in one dimension).
15. C. Ochsenfeld, C. A. White, and M. Head-Gordon, Linear and sublinear scaling of Hartree-Fock-type exchange matrices, J. Chem. Phys., 109, 1663 (1998).
16. Switch on for large QM systems (several hundreds of atoms).
17. For further information (D2 or D3, damping function etc.) and other options see Q-Chem manual.
18. Set any additional input designated for the $rem-section of Q-Chem (see Q-Chem manual) by specifying
    

    add =	{	[rem_name1]	[rem_value1]
    		[rem_name2]	[rem_value2]
    		.
    		.
    		.
    	}

    Use with care and be aware, of course, that by this the Q-Chem output may be modified in such a way that the proper extraction of QM results may fail!

Examples