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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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4 votes
2 answers
110 views

Options for automated Q-Chem input file generation

I have a big number of .xyz files which I plan to run using Q-Chem, and I was wondering if there were any options for automatically generating Q-Chem input files from these .xyz files. I know that ...
6 votes
2 answers
115 views

Is there any way to read in orbitals from Turbomole into Q-Chem?

The lack of interoperability of quantum chemistry software is one of the main issues holding back the field. Unfortunately, not all programs implement every possible type of calculation, and often one ...
3 votes
0 answers
174 views

Advise in generating ESP surfaces from Gaussian .cube files using the IQmol software

I am trying to generate an ESP surface of a Gaussian09 calculation using the iqmol software. I have used Gaussview to visualize the ESP surface by generating .cube files from .fchk files (using the ...
5 votes
0 answers
271 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 ...
18 votes
1 answer
419 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&...
11 votes
2 answers
445 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 ...
17 votes
3 answers
934 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 ...
12 votes
1 answer
620 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 ...
16 votes
3 answers
1k 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 ...
14 votes
2 answers
908 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
2 answers
191 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 ...
12 votes
1 answer
106 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
1 answer
289 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 ...
11 votes
1 answer
90 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
1 answer
208 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 ...
19 votes
2 answers
393 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 ...
13 votes
2 answers
652 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,...
10 votes
2 answers
391 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, ...