Questions tagged [quantum-chemistry]

For questions about quantum chemistry.

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Calculation of S1 energy in Gaussian

I am trying to calculate the energy of the S1 state with UDFT in Gaussian software. I understand that, generally, one should manipulate the charge and spin multiplicity to manually change the ...
Laura's user avatar
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2 votes
1 answer
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'The combination of multiplicity electrons is impossible' in Gaussian

I am trying to calculate the energies of the S1 and T1 states of a molecule with UDFT using Gaussian. For the T1 state, I can successfully use the following input file: ...
Laura's user avatar
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4 votes
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How to Model Cations or Anions for Activation Energy Barrier Calculation in VASP for Electron Exchange Reactions?

I am interested in performing activation energy barrier calculations using VASP (Vienna Ab initio Simulation Package) specifically for reactions involving electron exchange. However, I am uncertain ...
Nidhi Tiwari's user avatar
7 votes
1 answer
333 views

Have quantum computers been proven to speed up any matter modeling algorithm?

Personally, I think quantum chemistry is one of the most promising applications for quantum computers because the problem itself is fundamentally quantum, so there is an obvious argument why we should ...
taciteloquence's user avatar
5 votes
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What are the most promising areas where quantum computers could advance quantum chemistry in the next 3-5 years?

A very similar question was asked about 4 years ago, but this is a fast moving field, so I think it warrants reexamining. What are the most promising areas where quantum computers could advance ...
taciteloquence's user avatar
1 vote
1 answer
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How to use the CI method to compute the order of the molecular orbitals of the nitrogen molecule

It is well known that the Hartree–Fock (HF) approximation predicts a wrong order for the last two molecular orbitals of N$_{2}$. The calculations indicate that the 1$\pi_{u}$ orbital has higher energy ...
AlfredV's user avatar
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1 answer
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Geometry of H2O in the v=1 vibrational state for AIMD simulation

This is a follow-up to my recent question: Algorithm for finding the v=1 state of an H2O molecule I want the geometry (atomic positions) of the H2O molecule in the v=1 state. How are initial ...
Nidhi Tiwari's user avatar
1 vote
1 answer
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Algorithm for finding the v=1 state of an H2O molecule

In the context of simulating the dissociation dynamics of an H₂O molecule on a metal surface, what algorithm is employed for generating the initial state of a tri-atomic non-linear molecule, such as H₂...
Nidhi Tiwari's user avatar
4 votes
1 answer
79 views

How to orthonormalize a set of Molecular orbitals?

I am using PySCF, and checked that the molecular orbitals(MO) from the HF calculation are orthonormal. If S is the Overlap matrix, and V is the matrix of the MO coefficients, It can be seen from the ...
Pro's user avatar
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7 votes
2 answers
157 views

Why is electron correlation called the "chemical glue"?

A significant part of quantum chemistry involves developing methods that go beyond the Hartree-Fock theory which treats electron-electron interactions through a mean-field approach. From what I ...
Fracton's user avatar
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1 answer
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How do I determine the charge and spin multiplicity of a system?

I have a compound (5c from here) as below which I wish to optimize in ORCA. However, I am not sure what is its charge and multiplicity. This is an Ir(III) complex. Iridium has a valence configuration ...
farmaceut's user avatar
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Reproduce Dissociation Energy of BH molecule with PySCF

I want to reproduce the bond dissociation energy of the BH molecule as stated in the book 'Molecular Electronic Structure Theory' by Helgaker et al, (chapter 8.3.1), using FCI with 1 frozen core ...
jakg's user avatar
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7 votes
1 answer
71 views

Numerical atomic orbitals vs Gaussian-type orbitals for molecular calculations

Gaussian-type-orbitals (GTOs) are the work horse of modern molecular quantum chemistry calculations. They are computationally efficient and allow for analytic evaluation of integrals. However, there's ...
Fracton's user avatar
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2 votes
1 answer
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How can I calculate ionization energies and electron affinities with MOPAC?

can I simply calculate the ionization energy (IE) as the difference between the neutral and the charged system Heat of formation (Hf) with MOPAC ? The same question is with the electron affinity (EA). ...
Miro Iliaš's user avatar
3 votes
1 answer
86 views

How may I verify that my A matrix for TDA (Tamm-Dancoff Approximation of TD-DFT) from PySCF is correct?

This is a follow-up to a series of questions that I have asked here: Is a D-Wave quantum computer able to do TD-DFT calculations? (answer was "yes, but it might not be efficient") Inquiry ...
Hadeel Moustafa's user avatar
2 votes
0 answers
14 views

Can I define molecular units on NBO3.1 or NBO6.0?

I'm studying donor-acceptor charge transfer (CT) process between tetrakis(m2-acetato)-di-Wolframium (acceptor) with an acetonitrile (donor) at the B3LYP-D3BJ/def2-TZVPD level of theory. The W···N ...
Juandiego Velasquez Benites's user avatar
2 votes
0 answers
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How to set up a calculation including dummy atoms in Dalton Quantum Chemistry Package?

I want to plot a magnetic shielding isosurface around benzene molecule, for that, I prepared a 100 atom grid keeping benzene at the center. At each grid point there is a dummy atom (X with charge=0.0)....
MeliodusSama's user avatar
6 votes
1 answer
92 views

Is it essential to include solvent cage effect in radical reactions?

My question is straightforward and is outlined in the post title: Is it essential to include the solvent cage effect in radical reactions during the computational estimation of Gibbs free energies and ...
farmaceut's user avatar
4 votes
0 answers
21 views

How to evaluate excited state MO coefficients using Dalton Pacakge?

I need the excited state (T1) wavefunction associated with my benzene molecule, but even after trying multiple times using multiple input formats, I am not able to find the correct input to evaluate ...
compcal's user avatar
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5 votes
1 answer
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A matrix form used in the Quantum Annealer Eigensolver (QAE)

In the paper "Computing molecular excited states on a D‑Wave quantum annealer", PySCF is employed to compute the TDA matrix. Then a few of the lowest eigenvalues and eigenvectors of a TDA ...
Hadeel Moustafa's user avatar
4 votes
1 answer
112 views

Inquiry about A Matrix Output in 21-matrix_A_B.py

I'm trying to use the pyscf library for some calculations, specifically the 21-matrix_A_B.py example script for obtaining the A matrix. https://github.com/tepl/pyscf/blob/master/examples/tddft/21-...
Hadeel Moustafa's user avatar
4 votes
2 answers
95 views

Total energy calculation for an isolated atom

I tried calculating total energy using SCF calculations after optimization with quantum espresso. Still, I don't know how to optimize total energy in the standard state instead of isolated atoms. If ...
MALAK's user avatar
  • 43
4 votes
2 answers
128 views

Post-hartree fock calculation with localized Molecular orbitals in PySCF

I have localized the canonical Hartree-Fock orbitals for a molecule in PySCF. Now I want to perform ccsd calculations with these localized orbitals. Is it possible with pyscf? Example with CCSD with ...
Pro's user avatar
  • 934
3 votes
1 answer
174 views

Is a D-Wave quantum computer able to do TD-DFT calculations?

I was reading this paper (Computing molecular excited states on a D‑Wave quantum annealer) and wondering if I can make a QUBO model for the equations used in the TD-DFT energy calculation for the ...
Hadeel Moustafa's user avatar
3 votes
1 answer
105 views

What are the observables when a quantum system is comprised of orthogonal spacetimes? [closed]

We are given a system of three quantum domains A, B, and C with the following properties: (1) domains A and B are independent Hilbert spaces, each with their own space and time dimensions. Both are ...
RalphW's user avatar
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4 votes
1 answer
120 views

Spin-orbit coupling and lattice constant

Cross-posted on Chem.SE. Is there any evidence that spin-orbit coupling (SOC) leads to a crystal lattice distortion? Using Quantum Espresso and DFT, I am getting different relaxed lattice constants ...
freude's user avatar
  • 382
5 votes
1 answer
59 views

Is it possible to predict reduction reaction products with a Gaussian program?

In a molecular geometry calculation using a Gaussian program, can I see changes in bond lengths or changes in structure when one electron enters the molecule through a reduction reaction? for example, ...
Jay Kim's user avatar
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7 votes
0 answers
159 views

How do you calculate a full quartic force field for anharmonic frequencies?

I am an experimental chemist interested in anharmonicity—I am particularly interested in how to calculate anharmonic frequencies. The method I have the most experience with is (generalised) second ...
isolated matrix's user avatar
2 votes
1 answer
92 views

ORCA Error: NCalcs not set

I've been getting the following error whenever I use CCSD(T) and the ORCA Forums don't have an answer to this one: ...
znc204's user avatar
  • 63
4 votes
1 answer
401 views

Kill or let be: Is this calculation stuck or just very slow?

My question is regarding a medium-to-large DLPNO-UCCSD(T)-F12 calculation. I have a ~250 electron system, an open-shell molecule inside a water droplet, and I'm trying to get the total energy, first ...
epalos's user avatar
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9 votes
1 answer
3k views

Software for quantum electrodynamics?

I've been reading some beginner books about quantum electrodynamics / quantum field theory (e.g. Feynman's QED book), and basically everything is done at the paper and pencil level. I'm curious if ...
Alex I's user avatar
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1 vote
0 answers
25 views

Change bonding thresholds in NBO Calculations? [closed]

I'm looking at a system with really long bond lengths, and I'm trying to determine if there's any interaction at all between orbitals on two particular atoms. I'm using NBO7 with the GENNBO interface, ...
NicoleJ's user avatar
  • 11
5 votes
1 answer
167 views

Is it okay to use semi-empirical methods to optimize structures if I perform high-level energy calculations?

For non-covalent systems comprised of small-molecule monomers, folks are usually interested in understanding the interaction and binding energies of small (gas-phase) clusters ... things like $\ce{X(...
epalos's user avatar
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3 votes
1 answer
183 views

Comparison between the ordering of Molecular Orbital coefficients between Gaussian and PySCF

I am trying to compare the Molecular Orbital coefficients obtained from PySCF and G09 program. Below is the PySCF and Gaussian input and output. I am using 3-21G basis and RHF method for both the ...
Pro's user avatar
  • 934
8 votes
3 answers
133 views

Analytical derivative of nuclear-electron attraction integrals over gaussian functions

I'm currently writing a program which evaluates the first derivatives of molecular integrals (over gaussian basis sets) with respect to the cartesian coordinates of the nucleus. I'm somewhat stuck ...
dav267's user avatar
  • 153
6 votes
1 answer
309 views

Which is better for binding energies: DFT-D3 or RI-MP2?

I am trying to calculate the binding energy of a small molecule with an underlying surface. I have used B3LYP-D3BJ (as implemented in ORCA) at def2-TZVP basis set to optimize the geometry of the VdW ...
Hemanth Haridas's user avatar
3 votes
1 answer
347 views

Repeated Error message ORCA

I am repeatedly getting the error message: [file orca_main/maininp1.cpp, line 17547]: ERROR: expect a '$', '!', '%', '*' or '[' in the input when I run my Orca script. My inp file looks like: ...
Madeleine Fisher's user avatar
9 votes
1 answer
781 views

Why is chemical bond dissociation difficult to simulate?

I'm a student doing quantum computing research trying to understand better what kinds of chemical systems are difficult to simulate on a classical computer and for which quantum simulation could be ...
Solarflare0's user avatar
4 votes
0 answers
47 views

Why, if the potential is different from the Coulomb one, but has spherical symmetry, the eigenvalues of the system are non-degenerate? [closed]

Question duplicated from here https://mathematica.stackexchange.com/questions/287433/why-if-the-potential-is-different-from-the-coulomb-one-but-has-spherical-symme and here https://physics....
Mam Mam's user avatar
  • 253
1 vote
0 answers
24 views

NLMO algorithm failed to converge [closed]

I have been using NBO 7.0 software and the file.47, and the option in ORCA with nbo block and sometimes I have problems with the convergence algorithm for NLMO specifically this error: NLMO algorithm ...
William Quintero's user avatar
4 votes
0 answers
47 views

How to compute only part of molecular Hessian accurately (hybrid Hessian or partial Hessian calculation)?

Hessian is required for minimisation and transition state (TS) optimisation algorithms. While the full Hessian at a certain level of theory is calculated usually for TS optimisation, usually ...
S R Maiti's user avatar
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6 votes
0 answers
49 views

PC Specifications for a small DFT Research Group [duplicate]

Planning to set up a small research group here at my university to do some DFT/MD work and I'm looking for a nice setup that will cater to that. Will be asking for funding from the university or from ...
tumblewush's user avatar
3 votes
2 answers
67 views

How to remove f functions from a cc-pVQZ basis set in CFOUR format?

I need to perform a calculation on a BN system with using cc-pVQZ stripped of the f functions. However, I don't really know how to do this and I have not been able to find a revised one already. I ...
farmaceut's user avatar
  • 536
4 votes
0 answers
145 views

Help with Mathematica for optimizing a Gaussian basis set for a hydrogen atom in a magnetic field

This is a follow-up to my previous question: Optimal Gaussian basis set for hydrogen atom in magnetic field Brief description of the problem I would like to find the ground and excited states of the ...
Mam Mam's user avatar
  • 253
6 votes
1 answer
205 views

Compute polarizability after each step of relaxed scan in Gaussian 16

I am computing a relaxed 2D scan with Gaussian 16 in order to reproduce some computational data. I am interested in computing the dipole moment vector and the (static) polarizability matrix after each ...
capitn96's user avatar
5 votes
1 answer
123 views

A terminology for QM calculation methods

I wonder if there is any terminology (or name of classification) that refers to ab-initio QM methods based on the Schrodinger equation and basis functions, such as HF and post-HF methods, but not DFT ...
nonon's user avatar
  • 53
7 votes
1 answer
157 views

Optimal Gaussian basis set for hydrogen atom in magnetic field

Copy from here https://mathematica.stackexchange.com/questions/285044/optimal-basis-set-of-gaussian-functions-for-describing-a-quantum-system This question arose during the discussion of the previous ...
Mam Mam's user avatar
  • 253
5 votes
0 answers
93 views

How can one determine the time-step for imaginary time evolution?

The real-time evolution of a quantum system described by the state $\psi(t)$ is formally given by, $$ \psi(t+\Delta t) = \exp^{- i \hat{H} \Delta t / \hbar} \psi(t)\tag{1}$$ For dynamical simulations ...
squareroottwo's user avatar
6 votes
1 answer
145 views

Scanning two bond lengths G-16

If a scan is performed over two CVs, or collective variables (bond lengths here), does the code simultaneously step through both of the CVs or does it step through the first one, and scan the second ...
steve kukolich's user avatar
9 votes
1 answer
675 views

Usual way of computing exchange-correlation potential on a simple case H2 (RKS, LDA)

The python code is here. I tried to do a DFT calculation for H2 Restricted Hartree-Fock with Local Density Approximation. The optimization is a scf-loop with Lagrange-multiplier only. I use the same ...
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