DX in vibrational analysis

[Tobias Kraemer]

Dear all,


following my previous post (huge number of imaginary frequencies...) on 
this issue, I have in the meantime
tried to break down to problem into a smaller one.Basically, instead of 
using the full bulk system, I have tried
a simple optimization and frequency calculation on one of the cationic 
portions (organometallic complex) of the
system. I have used a cubic cell with a = 20 Ang. I have made sure that the 
structure for the subsequent vibrational
analysis job does indeed correspond to the final structure of the 
optimization run (I have included ATOMIC_COORDINATES ON
to print out the geometry in the output).  Again, in the first attempt 
(using DX 0.001) I end up with a huge number (~50) of imaginary
frequencies, including some very large ones.

 VIB|                        NORMAL MODES - CARTESIAN DISPLACEMENTS
 VIB|
 VIB|                         1                    2                    3
 VIB|Frequency (cm^-1) -4133.188102         -2206.847291         
-2036.718924
 VIB|Intensities           0.326216             0.395802             
0.125908
 VIB|Red.Masses (a.u.)     1.746454             1.699628             
1.961778
 VIB|Frc consts (a.u.)    -1.304878            -0.103208            
-0.086426


Now, if I repeat the same calculation with a larger DX of 0.01, I end up 
with less imaginary modes, but still too many (17) to trust the result.
Also, massive change now in the magnitude of the wavenumbers, which drops 
to -957 cm-1 for imaginary mode nr 1. Think the details here
are not so relevant, but I am quite confused. So the problem is not related 
to the bulk system, but intrinsically related to the vibrational
analysis protocol I am following at the moment. Also, as far as the SCF is 
concerned, I am using inner / outer SCF  in combination with DIIS.
Apparently the very first SCF never converges after 100 steps, which is 
when the outer SCF kicks in and from there everything seems to converge
smoothly. Maybe my thinking is wrong here? This is what it looks like (in 
condensed form) from one of the replica outputs: 

rhdcpenba_mol_vib-r-9.out:  *** SCF run NOT converged ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     4 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     6 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     6 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     8 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     9 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     8 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     9 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in     9 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    10 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    10 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    13 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    13 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    14 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    16 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    14 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    14 steps ***
rhdcpenba_mol_vib-r-9.out:  *** SCF run converged in    15 steps ***

So my question really is, how do I choose an appropiate value for DX (and 
other parameters), and what is potentially going wrong here. I am 
absolutely clueless I must say, since I considered this as an 'easy' task. 
I feel that I am just simply doing something wrong, although I thought I 
was following standard procedure. I am keeping all other values such as SCF 
convergence and so on untouched, in order to not introduce any 
inconsistencies here. Would a more rigorous optimisation perhaps be helpful?


Thanks for your help, much appreciated. Files attached. 

Best

Tobi
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