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.

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10
votes
2answers
301 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 ...
16
votes
3answers
244 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 ...
5
votes
0answers
83 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 ...
12
votes
1answer
190 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 ...
14
votes
2answers
769 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. ...
11
votes
2answers
106 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 ...
16
votes
3answers
294 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 ...
11
votes
1answer
61 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 ...
13
votes
2answers
438 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,...
18
votes
1answer
267 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&...
10
votes
1answer
150 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 ...
13
votes
1answer
138 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 ...
12
votes
1answer
69 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 ...
10
votes
2answers
154 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, ...
19
votes
2answers
332 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 ...