Cannot find desired molecular orbital in orca ourput file error
Answers
Answer:
ORCA needs more memory in order to complete the calculation. Controlling memory is usually necessary for various wavefunction theory calculations (MP2, CCSD(T), CASSCF, MRCI etc.) or when calculating excited states (TDDFT, CIS, EOM-CC, CASSCF, MRCI) or complex molecular properties (Hessian, NMR chemical shifts or spin-spin couplings). Knowing how to control the memory properly often allows you to make more efficient calculations as this reduces batches in some part of the code.
Memory usage in ORCA is controlled by the %maxcore keyword where the user should specify the memory per core in MB that the program is allowed to use. The default maxcore setting is
Example:
! DLPNO-CCSD(T) def2-TZVP def2-TZVP/C
%maxcore 3000
%pal
nprocs 6
end
Here 3000 MB of memory per core is given to ORCA. Since the use of 6 CPU cores was requested this means that the total memory demand of ORCA is 6x3000=18000 MB = 18 GB
Make sure that the computing node has this total amount of physical memory available.
In general you should not ask for more than 75 % of the physical memory available (since ORCA occasionally will use more than the maxcore setting). So if the computer in this case has 24 GB or more of physical memory available, then things should be fine.
If the computing node you are using is also being used by other users make sure not to request more memory than you are entitled to.
Imaginary vibrational modes from a frequency job
Small imaginary modes (~ below 100 cm-1) like:
-----------------------
VIBRATIONAL FREQUENCIES
-----------------------
0: 0.00 cm**-1
1: 0.00 cm**-1
2: 0.00 cm**-1
3: 0.00 cm**-1
4: 0.00 cm**-1
5: 0.00 cm**-1
6: -70.85 cm**-1 ***imaginary mode***
7: -50.05 cm**-1 ***imaginary mode***
8: 48.60 cm**-1
9: 169.21 cm**-1
10: 176.59 cm**-1
11: 241.39 cm**-1
are typically indicative of numerical noise present. The noise causing this might be present in the Hessian alone or it might have been present in in the energy or gradient in the geometry optimization resulting in a suboptimal geometry. In this case, the problem is fixed by increasing the integration grid by adding "Grid4 FinalGrid5" keywords. If the RIJCOSX approximation was used in either the geometry optimization or the Hessian calculation, the numerical noise might also be due to the COSX grid. For more information, see Numerical precision page.
It may also be that you simply need to lower the thresholds for the geometry optimization a bit (maybe the energy surface is a bit flat) so that you converge more precisely to the minimum. TightOpt is a convenient way of doing that.