HERMIT is the integral evaluation part of the code. In ordinary calculations there is no need to think about integral evaluation, as this will be automatically taken care of by the program. However, HERMIT has an extensive set of atomic one- and two-electron integrals, and some users may find it useful to generate explicit integrals using HERMIT. This is for instance necessary if the RESPONSE program (dynamic properties) is to be used, as described in Chapter 26. Disk usage may also be reduced by not calculating the supermatrix, and this is also controlled in the **INTEGRALS input section.
It is worth noticing that the two-electron part of HERMIT is actually two integral programs. TWOINT is the more general one and is invoked by default in sequential calculations; ERI (Electron Repulsion Integrals) is a highly vectorized two-electron integral code with orientation towards integral distributions. ERI is invoked by default in integral-direct coupled cluster calculations and may in other cases be invoked by specifying the .RUNERI keyword in the **DALTON INPUT input section, which ensures that ERI rather than TWOINT is called whenever ERI has the required functionality. DALTON will, however, automatically revert to TWOINT for any two-electron integral not available in ERI: ERI cannot be used in parallel calculations and it contains only first-derivatives (for the Hartree-Fock gradient), thus it cannot be used to calculate Hessians or to calculate MCSCF gradients.
Input to integral evaluation is indicated by the keyword **INTEGRALS, and the section may be ended with *END OF or any keyword starting with two stars (like e.g. **WAVE FUNCTIONS). The intermediate input is divided into two sections: one general input section describing what molecular integrals are to be evaluated, and then a set of modules controlling the different parts of the calculation of atomic integrals and the (possible) formation of a supermatrix as defined in for instance Ref. [114].