CCL: Atomic masses used in Orca for computing vibrational frequencies

[Grigoriy Zhurko <reg_zhurko###chemcraftprog.com>]

 Sent to CCL by: Grigoriy Zhurko [reg_zhurko===chemcraftprog.com]
 > This is the standard atomic weight. An interval is given because the value
 depends on the origin of the material.
 > From the ORCA manual at https://www.faccts.de/docs/orca/6.0/manual/contents/typical/frequencies.html
 > It is clear that ORCA uses atomic weights to calculate the hessian. But
 this can be changed afterwards using the orca_vib routine. Or during the
 calculation with the !Mass2016 keywords, that commands Orca to use the isotopic
 masses of the most abundant isotopes.
   I performed an Orca computation with !Mass2016 keyword. It indeed prints the
 masses of most abundant isotopes (12.0 for C, 1.00783 for H, etc). However the
 frequencies computed by Orca and my program do not match. I found that Orca in
 this case uses the mass 12.0004364861161 for C. For calculating this value, I
 computed a C4 molecule and divided a frequency produced by my program by the one
 printed by Orca.
   I remember that Orca uses some QRRHO approach instead of RRHO; firstly I
 thought that this is applied only for computing the entropy, but maybe for the
 frequencies computations too?
   Also, I have one more question for all: would you find useful if Chemcraft
 computes the vibrational spectrum for each variant of isotopes distribution in
 the molecule, and averages them and shows at a singe graph (like in the
 computations of mass spectra)? I see that e.g. for Cl the isotope Cl37 produces
 different frequencies than Cl35, and its percentage is rather high - 25%.
   Grigoriy.