Questions tagged [q-chem]
Questions related to the ab initio quantum chemistry software package Q-Chem. Q-Chem includes DFT/HF calculations as well as high level post-HF correlation methods.
16
questions
19
votes
2
answers
383
views
What significant matter modelling methods are implemented in commercial software, for which there is no freeware alternative?
There is an ever-growing list of freeware and open-source software for solid-state physics and quantum chemistry.
But many commercial programs still thrive, even in 2020, and their cost can be in ...
- 291
18
votes
1
answer
391
views
How to set up a local server for iQmol on macOS?
I'm trying to set up iQmol as a tool to submit calculations to Q-Chem on our lab machines to make access to it easier for OChem students. Unfortunately, it is not working properly "from the box&...
- 1,658
17
votes
3
answers
722
views
How is "basis set projection" done?
I recently came across something called the basis set projection in Q-chem's manual. What it seems to do is to take the converged SCF in a smaller basis set, and then "projects" that onto a ...
- 6,436
16
votes
3
answers
905
views
How to calculate homolytic bond dissociation energies?
An experimental colleague asked me how hard it would be to calculate homolytic bond-dissociation energies for different phosphonates which are involved in a Hydrophosphination. The compounds include ...
- 7,937
14
votes
2
answers
868
views
What is the most user friendly software for frequency visualization rendering?
I perform my calculations in Q-Chem and would like to make short movies of IR modes. As of now, I render them with IQmol, but I believe graphics properties (colors and resolution) can be much better. ...
- 1,658
13
votes
2
answers
611
views
How to export a calculated Raman spectrum in Q-Chem, as a Gaussian profile?
Does anyone know how to export a calculated Raman spectrum (my calculation was performed in Q-Chem) as a data-file, which can be used to plot the profile of a Gaussian-shape spectrum (in origin, excel,...
- 1,658
13
votes
1
answer
181
views
TS-search with explicit solvent
I'm looking for an advice how the best to set up the system and optimize a transition state with explicit solvent.
I believe that solvent molecules are participating in the reaction by specific ...
- 1,658
12
votes
1
answer
522
views
Is there a software that can calculate collision cross-sections involving molecules with several atoms?
What software currently can be accessibly used to calculate collision cross section using the geometries obtained from DFT? In my case, Q-Chem is used for geometry optimization.
The goal is to compute ...
- 1,658
12
votes
1
answer
89
views
Q-Chem QM/MM options
I am trying to setup QM/MM calculations with QChem using user-defined connectivity and MM parameters. But the official manual is not detailed about some options. Mostly I am interested in the ...
11
votes
2
answers
174
views
Which SCF energy to use to plot PES after calculations with SMD?
I have a series of molecules calculated (DFT PBE0-D3/def2-TZVP performed in Q-Chem) and I'm ready to plot PES (Potential Energy Surface). I had to increase the grid to get rid of small imaginary ...
- 1,658
11
votes
1
answer
79
views
Parse linked frequency calculations with cclib
I have multiple Q-Chem output files for TS search starting and finishing with frequency calculations (two sets of calculated IR data in total). To parse and plot spectra I use cclib and a script ...
- 1,658
10
votes
2
answers
408
views
How to calculate DFT energy with density from another level of theory?
I need to calculate the DFT energy given the density of another level of theory, for example Hartree Fock.
$E[\rho_{HF}]$
This is sometimes used to fix various electron delocalization errors in ...
- 7,937
10
votes
2
answers
341
views
Why is Q-Chem producing a parallel efficiency greater than 1?
The efficiency $E$ of a parallel calculation is defined as
\begin{align}
E &= \frac{S}{N}, & S &= \frac{T_1}{T_N},
\end{align}
where $S$ is the speedup and $N$ is the number of workers, ...
- 7,937
10
votes
1
answer
270
views
When running DFT on Q-Chem with an HPC cluster, why is parallelization with OpenMP faster than with MPI?
How to maximize the efficiency/time-to-solution of the calculations in Q-Chem on HPC clusters?
MPI parallelization is supposed to improve time-to-solution as compared to OpenMP. The system I tested ...
- 1,658
5
votes
0
answers
214
views
Q-Chem gives "Hessian Appears to have all zero or negative eigenvalues" error when calculating Frequencies [closed]
I have troubles with obtaining spectra for selected molecules in different DFT functionals.
I calculate IR spectra for my target molecules (gold complexes with C/H/N/O/S/Au composition) to find how ...
- 1,658