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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5 votes
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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 ...
18 votes
1 answer

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
2 answers

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 ...
17 votes
3 answers

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 ...
12 votes
1 answer

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 ...
16 votes
3 answers

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 ...
14 votes
2 answers

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
2 answers

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 ...
12 votes
1 answer

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
1 answer

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 ...
11 votes
1 answer

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
1 answer

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 ...
19 votes
2 answers

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 ...
13 votes
2 answers

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,...
10 votes
2 answers

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, ...