ORCA Error: NCalcs not set

Question

I've been getting the following error whenever I use CCSD(T) and the ORCA Forums don't have an answer to this one:

[file orca_numfreq/numfreq_utils.cpp, line 421]: Error (ORCA_NUMCALC): NCalcs not set

The input file is as follows: 

!CCSD(T) DKH DEF2-TZVP FREQ VeryTightSCF

* xyz 0 1
Zn 0.000 0.000 0.000
*

Is there something missing here? Sorry, I am still relatively new to ORCA and DFT calculations. I am trying to calculate the ionization enthalpy of Zn, and I'm basing this on a paper.

Answer 1

There is no point of doing frequency calculations for a single atom, since atoms do not have vibrations, which means that they do not have vibrational frequencies. The enthalpies of single atoms can be trivially calculated by hand, from its definition: $$ H = E_{ele} + ZPE + (U(T)-U(0)) + pV $$ For atoms, and under the standard approximations, ZPE is zero (because there are no vibrations), U(T)-U(0) (the difference of internal energies at the target temperature and at 0 K) is completely contributed by translational degrees of freedom and amounts to (3/2)RT, and pV=RT. Therefore, to get the enthalpy of a single atom, you just need to add (5/2)RT to the electronic energy of the atom, $E_{ele}$. Note that this is also why the isobaric molar heat capacities of monoatomic gases are (5/2)R. Of course, if you are solely interested in calculating the ionization enthalpies, then things are even simpler: as the enthalpy corrections of both the neutral atoms and the atomic cations are (5/2)RT, they cancel out when calculating the ionization enthalpies, so that you don't even need to calculate the (5/2)RT correction.

Furthermore, as the paper you mentioned is quite old, it may be worthwhile to consider using methods developed (or gained popularity) in the past two decades instead, which gives you much more accurate energies at affordable cost. For example:

1. The electronic energies may be calculated using a selected CI, a DMRG, or a FCIQMC method, which give results that are much closer to full CI than CCSD(T) gives. Within ORCA, the most convenient choice may be the ICE method, which belongs to the category of selected CI methods, and has the extra advantage that its wavefunction has no spin contamination.
2. Explicitly correlated methods (e.g. F12 or transcorrelated methods) or complete basis set (CBS) extrapolations can be used to minimize the basis set error. ORCA supports both F12 and CBS extrapolations.
3. Spin-orbit coupling should be considered, since single atoms have unquenched orbital angular momenta, which may give a non-negligible energy contribution when they couple with the spin angular momenta.
4. If possible, and particularly if you want to study heavier elements, use X2C instead of DKH. X2C is not much more expensive than DKH, but provides essentially exact relativistic energies compared to solving the Dirac equation. For atoms you can even remove the word "essentially" - X2C is an exact method, and it acquires minuscule errors for molecules only because the inter-atomic blocks of certain matrices are typically neglected for efficiency reasons.