Questions tagged [quantum-chemistry]

Quantum chemistry is a subfield of quantum mechanics. Like its parent field, quantum chemistry focuses on understanding physical phenomena occuring at the atomic scale. Quantum chemistry however is more focused on providing useful descriptions of electronic structure to aid in understanding chemical problems (e.g. reactions, spectra, dynamics, ...).

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Evaluating Franck-Condon factors

Franck-Condon overlap integrals are given by $\langle\psi^e_{v_e}|\psi^g_{v_g}\rangle$, where $\psi^e_{v_e}$ and $\psi^g_{v_g}$ are the vibrational functions for the $v^{th}$ vibrational states of the ...
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What computational methods are available for predicting nanoparticle structure?

I'm a quantum chemist, and nearly every system I've ever cared about has been studied by DFT. However, I am currently trying to model the surface interactions on a nanoparticle around 100 nm in ...
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21 views

Identifying luminescent transitions for lanthanides

Using the ground-state of Eu(III), how can I identify the possible (number of) luminescent transitions from the $^{5}D_{0}$ excited state? I identified the ground-state term symbol for Eu(III) using ...
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42 views

Getting Molecular Orbital contributions to the ESP fitted charges

For a single determinant wavefunction, the electron density is the sum of the densities arising from each molecular orbital. The molecular Electrostatic potential $\phi_{mol}$ is calculated from the ...
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194 views

How to solve the Schrödinger equation of 1D hindered rotor?

I have the following equation: $$-\frac{{{\hbar }^{2}}}{2I{}_{r}}\frac{{{d}^{2}}\psi }{d{{\theta }^{2}}}+V\left( \theta \right)\psi =E\psi, $$ with $$V\left( \theta \right)=a+\sum\limits_{n=1}^{...
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45 views

Expression for an expanded configuration interaction wave function

I am trying to write down an expanded expression for the configuration interaction wave function. We know that the wave function $\Psi$ is a linear combination (LC) of configuration state functions (...
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117 views

Is there a physical explanation behind the derivation of spectroscopic term symbols?

In deriving the spectroscopic term symbols, one method seems common among several sources, the clearest presentation for me being located here, a derivation of the term symbols for a $d^2$ ion. So, ...
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197 views

Basic Intro to Quantum Chemistry. Two Slit Experiment and Bohr Model

We just learned about the 2 slit experiment in Quantum Chemistry today, where electrons behave as waves when nobody is looking and behave as particles when they are being observed. So, what would ...
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681 views

How to Locate the Excited State Dipole Moment in Turbomole Output (TD-DFT)

I want to calculate the dipole moment for the first excited state with a Q.C. program (TURBOMOLE: TD-DFT). The calculation lists the following output: ground state total dipole moment: ...
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36 views

Can we relate approximate Hartree-Fock orbitals to true solutions in the basis-set limit?

Assume that we've "solved" the RHF Hartree-Fock equations in some finite basis, that is, we have arrived at a self-consistent set of coefficients $c_{\mu{}i}$ such that each spatial ...
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39 views

Challenges Associated with Earth's field NMR

The ACS recently hosted a virtual conference and there was an interesting talk on NMR experiments using the Earth's magnetic field. What surprised me was the number of molecules studied with Earth's ...
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150 views

Generating hybrid atomic orbitals for Boron

My professor was showing us how to form hybrid atomic orbitals for Boron by taking a linear combination of rotated 2p orbitals and 2s orbitals (of Boron). This was done in a particular manner such ...
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99 views

What are the cheapest basis sets able to model intermolecular dipole interactions inside a metal complex and give good relative energies, with DFT?

As a example, If I have a square planar complex of a cation with both a weakly polar ligand (X) and strongly polar ones, like ammonia and water I expect a bit of stabilization due to formation of a ...
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57 views

Why does Larmor precession occur in NMR?

Larmor precession is the phenomenon that in NMR the spin (and by default also the magnetic dipole moment) of protons does not line up with the applied magnetic field but rather precesses around this ...
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59 views

Perturbational Molecular Orbital Theory - How to derive the results?

I am reading Albright's Orbital Interactions in chemistry. The qualitatively perturbational treatment of molecular orbitals seem to be a central tool used throughout the book. I've referred to the ...
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40 views

IRC analogue of conical intersections and minimum energy crossing points?

In my project, there are some spin forbidden organometallic reactions that involves both the triplet minima and singlet transition state (TS). Using the penalty method in qchem as well the method of ...
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144 views

Explaining hypervalency in the framework of molecular orbital theory

In preparing for teaching duties in an introductory course in inorganic chemistry, I seem to have made a digression into how we understand the hypervalent molecules. Just to re-iterate what this ...
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179 views

Good software/module in python to determine nuclear attraction integral in Slater type orbital basis sets

I am trying to solve a Hamiltonian of one electron and 10 stationary nuclear centers. here, the electron is treated quantum mechanically and nuclear centers are treated classically objects. The ...
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92 views

If a single electron fills a degenerate energy level, does it fill all degenerate orbitals at once?

For example, if a single electron fills the 2p orbitals. Since all orbitals are degenerate, there is no reason why there would be any preference for px, py, pz. Is the electron hence in a ...
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188 views

Oscillator strength of a transition

I am trying to compute the oscillator strength in a molecule. Therefore I've got its Energy level values plus both wave functions $\Psi_i$ and $\Psi_j$ for the transition I want to calculate the ...
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134 views

Hückel method for biphenyl

I am trying to calculate the energy levels of biphenyl (see picture below) using Hückel method. Since this is my first time using Hückel I am a little confused. Biphenyl has 12 carbon atoms, ...
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60 views

Does a constrained wave function method exist similar to constrained density functional theory?

I am trying to find out if a constrained wave function method exists for running quantum chemistry calculations. I know that constrained DFT exists and can be used to constrain the charge density to ...
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85 views

Calculating excitation energies of a molecule

I am trying to calculate the excitation energies for the first three transitions of a molecule which is made up by a chain of k phenyl rings, where k shall be between 1 and 8. The angle between each ...
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105 views

Bond order analysis in molpro

I'm trying to compute bond orders in a molecule, let's consider S2 as an example. I'm not entirely sure what's the right way to do that, but I assume NBO (Natural Bond Orbital) should give me what I ...
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173 views

calculate SPE using dummy/ghost atoms in NWChem

I have been making some failed attempts to calculate interaction energy of 2 monomers using a dummy/ghost atom in NWChem6.5. The code I was using to calculate the interaction energy is: ...
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288 views

Crystal Field Theory - Operator Equivalent Methods

I am a bit confused by some of the points of operator equivalent methods in crystal field theory. I would appreciate any help I could get with any of my questions. First, I am confused as to the ...
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55 views

The difference between bonding and unpaired single electron radicals

I was wondering as to why would two unpaired single electron radicals form a bond. So, I tried to compute the added ground state energies of two particles in two distinct, yet identical boxes, The ...
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89 views

Precision of non-adiabatic couplings

Let me ask a question about the differentiation of Non-Adiabatic Couplings (NACs), specifically the terms $D_{IJ}$ and $d_{ij}$. According to the Pyxaid paper, the set of $D_{IJ}$ computed via Slater ...
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82 views

Error hunting: Guidelines on detecting error cancellation and locating their sources as much as possible?

It is known in computational chemistry, when computing e.g. electronic ground states energy and geometric paramters (bond lengths, angles, torsions etc.) of a given molecule using an ab initio or DFT ...
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173 views

How to derive the electron density of Hooke's atom?

Hooke's atom has the following wave function: $$\Psi( \boldsymbol{r_1}, \boldsymbol{r_2}) \propto \left( 1 + \frac{|\boldsymbol{r_1} - \boldsymbol{r_2}|}{2} \right) \exp\left(-\frac{(r_1^2+r_2^2)}{4}\...
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64 views

Request for a graph of the combined probability density of the three p-orbitals

This started when i was wondering why p-orbitals have an unsymmetrical shape (which isn't clear to me yet). But then i came across Halliday/Resnick/Walker's Fundamentals of Physics. In it, they have ...
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909 views

Are multiple transition states possible? Criteria to choose the correct one?

I am studying a reaction using electronic structure methods using various software packages (ORCA,GAMESS, G09) and have found two possible transition structures between product and reactant. Both have ...
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28 views

How are class II force field cross-terms derived?

In some class II force fields, there are cross-terms that characterize bond-bond, bond-angle interactions and others. Example of equations below: $$\sum_{bond-bond}^{}k_{bb}(b-b_{0})(b^{'}-b^{'}_{0})$$...
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28 views

Potential energy surface in transition state theory

I would like to study the typical reaction from the transition state theory: $$\ce{AB + C <=> ABC^‡ -> A + BC}$$ Assuming $\ce{ABC^‡}$ is a colinear molecule, the potential energy surface of ...
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52 views

The true shape of p orbitals

I'm trying to model $\mathrm{p}$ orbitals using a 3D program. What confuses me is that there are 2 shapes of $\mathrm{p}$ orbitals. I'm not sure which one is the accurate model. And what is the ...
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56 views

How is the ΔL = ±1 selection rule derived?

I understand there are selection rules in spectroscopy and quantum mechanics which governs whether a transition happens or not. For example, in atomic spectroscopy, the total orbital angular momentum ...
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60 views

What is the precise meaning of “in-phase” (and “out-of-phase”) in this context?

I am currently studying the textbook Infrared and Raman Spectroscopy, 2nd edition, by Peter Larkin. In a section entitled Symmetry: Infrared and Raman Active Vibrations, the author says the following: ...
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41 views

Determining symmetry correction by looking at rotational quantum states

I'm trying to understand the relationship between symmetry correction and rotational quantum states, particularly in the case of dipoles with identical atoms. For an angular momentum quantum number $...
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48 views

What are the alternatives we have to the VEDA 4 vibrational energy distribution analysis software?

Said software is very useful when you need to assign the peaks in vibrational spectra to different vibrational modes found in a hessian calculation for a given molecule, as described in Jamróz, Michał ...
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29 views

Why is the transition (0,0,0) -> (1,0,1) observed in a gas phase IR spectrum of CO2?

Let ($v_1$,$v_2$,$v_3$) denote the vibrational state of $CO_2$. Why is the transition $(0,0,0)\rightarrow (1,0,1)$ observed when the trasition $(0,0,0)\rightarrow(1,0,0)$ (asymmetric stretch) is not ...
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31 views

Energy decomposition analysis. Are there schemes that actually splits up the steric term?

In quantum chemistry, there are many types of energy decomposition analysis (EDA) which partitions the interaction energy between two or more fragments into steric, polarisation, exchange and orbital ...
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228 views

What exactly are the graphs of orbitals?

In my QM class we are finding the wave functions of the hydrogen atom. In spherical coordinates, these wave functions are functions of 3 variables: $r,$ $\theta,$ and $\phi.$ My professor stated that "...
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57 views

Is Tkatchenko's MBD-NL method self-consistent?

I have a question regarding Tkatchenko's [1] new nonlocal many-body dispersion method (MBD-NL). Does anyone know if it's a self-consistent method? I read this paper many times but I'm still not sure. ...
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50 views

Observing bands which I shouldn't be!

I performed the iodine spectrum experiment to observe vibronic transitions. I heated the iodine crystals to produce vapors (at a little above room temperature) and observed absorption spectrum with a ...
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25 views

Is there a way to extract the diamagnetic and paramagentic components from NMR output from Gaussian16?

I'd like to be able to split the NMR value in Gaussian16 into its diamagnetic and paramagnetic terms. I'm coming over from ADF, where this can be done easily. Is there a straightforward command to ...
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47 views

Do I understand correctly that simulation volume influences relative permittivity calculations? If so, how is the result useful?

I am having a lot of trouble understanding the influence of volume on relative permittivity/static dielectric constant. I'm not a chemist, but am using molecular dynamics to calculate relative ...
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67 views

Available datasets to study using Hückel theory

I'm trying to do some tests using graph Laplacians based on Hückel theory. This, however, is said to be only applicable for conjugated hydrocarbons. My questions are: I have been working with QM9 ...
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80 views

Using a the particle in a box - one dimension, to calculate the zero point energy of an hydrogen atom

I have been tasked with using the one dimensional particle in a box model to calculate the zero point energy of an hydrogen atom with different sizes. Using the formula $E_1=h^2/(8mL^2)$ for a box ...
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83 views

How to formulate the 1 dimensional time-dependent Schrodinger’s equation from a given function?

Assuming $f(t) =e^{−iωt}$, formulate the 1 dimensional time-dependent Schrodinger’s equation. I started solving the problem as \begin{align} E &= hf, \tag1\label{eq:1}\\ E &= ħw, \tag2\...
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49 views

How to account for solvation of a molecule explicitly in quantum calculation?

I'm looking for a way to account for hydrogen bonds formed with my molecule in aqueous solution explicitly in DFT calculation. I'll do it in order to compare stability of two structures in water. As ...