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
  • $\begingroup$ +1. Since you have H2O in the geometry section, by fragments do you mean {H2,OH} and/or {H,O,H}? If so, I'm pretty sure you would need to calculate the energy of H2,OH,O, and H separately (e.g. new input file and new run). $\endgroup$ Feb 16 at 17:53
  • $\begingroup$ No @NikeDattani. I mean the fragment OH and H. Unfortunately I can't share the "exact" geometry, so here the water molecule is just an example $\endgroup$ Feb 16 at 19:19
  • $\begingroup$ Usually you would have to do three separate calculations, with three separate input files: 1 calculations for OH, one for H, and one for H2O. How can an H2O calculation give you energies for OH and/or H? The integrals are different, and the SCF is different, and even the spin multiplicity is different (H is a doublet and H2O is a singlet, so H would be done with unrestricted SCF or restricted-open-shell-SCF whereas H2O would by default be done with restricted SCF). $\endgroup$ Feb 16 at 20:00
  • $\begingroup$ The documentation doesn't have the (1) and the (2) in the xyz file for an H2O calculation, neither here nor here nor here, nor in any of the CPCM pages such as this one and this one. Can you provide a link to documentation that has the parentheses? $\endgroup$ Feb 16 at 21:15
  • $\begingroup$ @NikeDattani I quote the manual "Fragments can be conveniently defined by declaring the fragment number a given atom belongs to in parentheses “(n)” following the element symbol (see 9.2.1)." $\endgroup$ Feb 16 at 21:20

3 Answers 3


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.

  • $\begingroup$ Thanks for the complete answer, unfortunately, I'd rather use ORCA, since all previous calculations have been made with this software. $\endgroup$ Feb 16 at 21:24
  • $\begingroup$ Does the FMO method give you what you want? It's a way to calculate energies of huge molecules by splitting it up into fragments and doing calculations on those, but in your case you just want the energies of the fragments? Also, the FMO method will give you an approximate B3LYP energy for the full molecule, whereas based on your question I assumed you wanted to get the true B3LYP energy of the fragments? $\endgroup$ Feb 16 at 21:29
  • $\begingroup$ It kinda does yes @NikeDattani. I'm a bit upset only because ORCA does not have it implemented $\endgroup$ Feb 17 at 9:08

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 describes various keywords for setting what methods are used to compute inter/intra fragment energies.


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)

    QM2CustomMethod "B3LYP"
    QM2CustomBasis "def2-svp"
    QMAtoms {0:1} 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.

  • $\begingroup$ Certainly, this will work for small systems. Doing this for big system will be unviable. $\endgroup$
    – Camps
    Mar 17 at 13:46

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .