Energy of each fragment in ORCA?

Question

After the definition of the fragments in the geometry section of the calculation, is there a method to call, or a keyword to turn on, in order to obtain the energy of the fragments themselves?

! def2-svp b3lyp cpcm(water)

*xyz 0 1 
O (1) 0 0 0
H (1) 0 1 0
H (2) 0 1 1
*

Answer 1

With ORCA, I don't know but...

You can use the GAMESS software.

From the manual:

The code for the Fragment Molecular Orbital (FMO) method has been a part of the standard GAMESS package since May 2004. The FMO method is the successor of the EDA scheme developed by K. Kitaura and K. Morokuma (known in GAMESS as Morokuma-Kitaura decomposition), however, the FMO code was written independently. In GAMESS only the full FMO method is incorporated whereas in the literature one can also find a simplified approach suited for molecular crystals. Since "FMO" is also used to mean "Frontier Molecular Orbitals" and the concept of fragments is also introduced in the EFP method (see above), it is stressed here that the FMO method bears no relation to either of the two methods, that is to say, it is independent of the two, but might be combined with either of them in the future just as EFPs are used in e.g. RHF.

In the Fragment Molecular Orbital (FMO), your system can be divided in fragments (automatically or manually). As FMO is deeply interconnected with the energy decomposition analysis (EDA) the method will return the energy of each fragment.

Within GAMESS, you can use different level of theory like Hartree–Fock, Density functional theory (DFT), Multi-configurational self-consistent field (MCSCF), time-dependent DFT (TDDFT), configuration interaction (CI), second order Møller–Plesset perturbation theory (MP2), and coupled cluster (CC).

Finally, to prepare the GAMESS input for FMO calculation, the software FACIO, fu-suite and/or FragIt can help.

Answer 2

I don't have ORCA to test this, but it looks like the LED (local energy decomposition) keyword, described in section 8.16.1 of the ORCA 4.2.1 manual would give you the energy of each fragment (as well as interaction energy between fragments).

If that doesn't work for your case, section 9.12.4.2 describes various keywords for setting what methods are used to compute inter/intra fragment energies.

Answer 3

I solved the problem using the QM/QM2 method provided inside ORCA. To obtain the energy of the two fragments I set both DFT levels to B3LYP and then used the QMAtoms to divide the "fragments"

! QM/QM2 def2-svp b3lyp cpcm(water)

%qmmm
    QM2CustomMethod "B3LYP"
    QM2CustomBasis "def2-svp"
    QMAtoms {0:1} end
end

*xyz 0 1 
O 0 0 0
H 0 1 0
H 0 1 1
*

With this calculation, I was able to obtain the two energy needed: whole system energy "low" level, and "high" level fragment energy. The difference between the two energies is the "low" level fragment's energy.