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Direct methods

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The entire SCF and Kohn-Sham DFT part of the DALTON code is direct, including all derivative two-electron integrals, and all the way up to the cubic response function. To perform a direct calculation, all that is required is to add the keyword .DIRECT in the general input section, as indicated in the following input example for the calculation of nuclear shieldings in a direct fashion:

**DALTON INPUT
.RUN PROPERTIES
.DIRECT
**WAVE FUNCTIONS
.HF
**PROPERTIES
.SHIELD
**END OF DALTON INPUT

By default the two-electron integrals will be screened [89], using an increasingly tighter integral screening threshold during the SCF iterations. Thus, direct calculations calculated using integral screening will, when converged, be almost as exact as the results obtained with integral screening turned off. The thresholds can be changed with the keywords .IFTHRS and .ICEDIF in the *TWOINT section of the **INTEGRALS input module. To turn off integral screening in direct or parallel calculations altogether, .IFTHRS should be set to 20 or larger.

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Dalton Manual - Release 1.2.1