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For questions seeking answers deduced from or composed of theories regarding the chemical substances and about models and methods of theoretical chemistry.

3
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0answers
31 views

What is the difference between non-adiabatic coupling and diabatic coupling?

The diabatic couplings are the off-diagonal terms of wave functions written in a diabatic basis. There is a need to use the diabatic basis when there is a breakdown of the Born-Oppenheimer ...
5
votes
1answer
111 views

How do restricted open shell calculations mathematically proceed in the context of the self consistent field iterations?

I'm specifically concerned with HF theory. I understand restricted closed shell calculations use the Roothaan equations. If I understand correctly these equations assume each filled molecule orbital ...
2
votes
0answers
29 views

How are imaginary frequencies and force constant calculated by DFT?

Computational calculations allow us to simulate the frequencies of molecules. They can even tell us if the optimized structure is a minimum, a saddle point or a maximum according to the number of ...
1
vote
0answers
45 views

The nature of Chemical Bonds [closed]

It was nearly a year ago when I appeared for my first interview through the KVPY (a research oriented scholarship exam) when one of the questions the panel asked me was : What is a chemical bond? ...
2
votes
0answers
29 views

Franck-Condon factors in photoelectron spectroscopy

Franck-Condon factors have been used to predict / assign the vibrational structure of photo-electron spectra. $FCF=|\langle\psi''_{vib}|\psi'_{vib}\rangle|^2 = |\langle\psi''_{vib,1}\psi''_{vib,2},\...
2
votes
0answers
28 views

How to know about radical character of a diradical molecule?

I understand that the radical character is quantified by the occupation number of the lowest unoccupied natural orbital obtained from UHF calculations or from CASSCF calculations. My issue is that I ...
4
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0answers
54 views

Why is spin-contamination undesirable?

Why is spin-contamination, introduced by e.g. Unrestricted Hartree Fock, undesirable, and to which problems further on can it lead? The only problem I currently see myself is that spin-contamined ...
10
votes
1answer
183 views

Why is density fitting not used to reduce four-center integrals further down to two-center integrals?

In density fitting, why don't researchers turn all the 4-center integrals into 2-center ones instead of stopping at composing them from 3-center integrals?
4
votes
1answer
104 views

Can we predict with any reliability whether a chemical is carcinogenic in people?

I know that some chemical compounds have been known to be carcinogenic in humans. But can we make a reliable prediction of how likely a substance is to be carcinogenic based on the chemical and ...
3
votes
1answer
90 views

Where can I get potential energy surface for simple molecules?

I'm looking for potential energy functions for simple molecules, such as water or hydrogen sulfide. I only need the region around the equilibrium geometry, preferably as a Taylor series expanded in ...
4
votes
1answer
87 views

Knowing that CI is variational for any basis set, why is coupled-cluster variational in the full correlation limit?

This is the question asked in Martin's comment to my answer to: Why is Coupled Cluster not variational? In the comment I was also asked to provide a publication as a reference:
6
votes
1answer
93 views

Orthogonality of wavefunctions for different normal modes

Vibrational wavefunctions corresponding to different vibrational levels of the same normal mode are orthogonal because they are eigenfunctions of the harmonic oscillator Hamiltonian, which is ...
5
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0answers
91 views

Vibrational Self-Consistent Field

There are a few things about VSCF that I don't understand: Which coordinates do you work in? If you work in normal mode coordinates, how do you evaluate the effect of the average of one normal mode ...
6
votes
1answer
94 views

Why is Coupled Cluster not variational?

It has been noted in several sources (e.g. ref. 1) that one of the disadvantages of the Coupled Cluster method for the description of electron correlation is that, since $e^T$ in $E_{CC} = \langle0|e^{...
4
votes
0answers
44 views

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 ...
9
votes
2answers
125 views

Are such high oxidation states as reported by Vojovodic et al. possible?

I am a student in theoretical chemistry and I am confused about the paper: Trends in adsorption of electrocatalytic water splitting intermediates on cubic $\ce{ABO3}$ oxides (Montoya, J. H.; Doyle, A. ...
4
votes
1answer
72 views

The CASSCF orbitals

I have two questions about the orbitals that we get after the CASSCF calculations. First, what are the CASSCF orbitals that we get from these calculations? I mean after optimization, in the CASSCF ...
4
votes
1answer
66 views

Maximum required order for the Boys function

I am planning to pretabulate a lookup table in order to evaluate the Boys function. We can expand the Boys function as: $$F_n (x_t + \Delta x) = \sum _{k=0}^\infty \frac{F_{n+k}(x_t)(-\Delta x)^{k}}{...
19
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5answers
2k views

Why do some chemical reactions require many steps?

I posted the following question in Physics SE and was advised to transfer it to Chemistry SE. I studied physics in college ten years ago and I recently started to learn biochemistry. I enjoy finding ...
3
votes
1answer
37 views

Understanding elementary rate laws from a probability stand point

I am learning about chemical kinetics and dynamics and as I understand for a general chemical reaction $$\ce{aA + bB -> cC + dD} $$ whose reaction rate, r, can be described by an elementary rate ...
1
vote
1answer
52 views

Chemical Kinetics- Net Number of Particles

I have a box of 5 types of particles, $A$, $B$, $C$, $x$ and $y$. They react in the following way. $\ce{$A$ + $B$ -> $C$ + $x$}$ and $\ce{$C$ + $y$ -> $A$ + $B$}$ If only the first reaction ...
6
votes
1answer
105 views

Spin in meta-GGAs

In density functional theory (DFT) we know that the local density approximation (LDA) leads to an exchange-correlation energy kernel of the form: $$E_\mathrm{xc}^\mathrm{LDA} =\int e_\mathrm{xc}\left(...
6
votes
1answer
117 views

Relationship between Hartree-Fock and Hückel coefficients

I am new in computational chemistry. I'm studying the Hartree–Fock–Roothaan (HFR) method and have some of questions about the HFR basis function coefficients. Do we have the condition on HFR ...
5
votes
0answers
55 views

MCSCF Hessian transformation

I have tried doing the transformation of the Hessian in MCSCF following the notation that is used in Molecular Electronic-Structure Theory (MEST). Here I try to show what steps I have taken (to get ...
3
votes
1answer
93 views

What are the similarities and differences of adiabatic and diabatic states?

The idea of an adiabatic potential energy surface or adiabatic state is closely related to the Born-Oppenheimer approximation. This has been discussed on this site before in some detail. People ...
1
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0answers
63 views

Whats the symmetric, anti-symmetric and non-symmetric part of the direct product of irreducible representations?

I am trying to understand how to see if a vibrational mode is Jahn-Teller active or not. According to the group theoretical description of the Jahn-Teller effect one needs to check if the symmetric ...
1
vote
1answer
23 views

Which are the major truths of paradigms on which the whole body of chemistry is built? [duplicate]

For example, as evolutionary theory is fo biology or the existence of numbers for math, the existence of an independent reality for physics, etc.
5
votes
1answer
59 views

Accuracy benchmarks for atomic multipole moments

Thermochemical accuracy is regularly quoted as 1 kcal mol$^{-1}$, or 4.2 kJ mol$^{-1}$. I'm expressing some data and I want to use benchmarks like this to validate them, however, they are not all in ...
5
votes
1answer
77 views

Number of water molecules in a box to simulate liquid water [closed]

I have seen that depending on the simulation approach, different number of water molecules are employed for studying liquid water properties. 32 in early DFT calculations The electronic structure of ...
14
votes
1answer
929 views

Can a zeroth order reaction be reversible?

After recently studying about chemical equilibrium, I was convinced that the forward and backward reaction rates meet each other at equilibrium. However thinking about zeroth order reaction annoyed ...
1
vote
0answers
30 views

What's a good chemistry book for a practical man [closed]

Yesterday I picked up my old organic chemistry book and started to read it from the first chapter. After a couple of minutes I realized that I don't understand the concepts at all. The first chapter ...
6
votes
0answers
59 views

Truncated CI Wavefunction

In a calculation of for example a Li-Atom in a DZ basis we have 4 MO. For the alpha string (containing two electrons), that means we have for a FCI calculation the determinants: |12>, |13>, |14>, |...
3
votes
1answer
209 views

Size consistency in computational chemisty methods

I am looking for a clear answer that would explain what exactly size consistency is, why configuration interaction (CI) is not size consistent and how this leads to errors when using CI for large ...
6
votes
1answer
164 views

Number of atomic and molecular orbitals in different Hartree-Fock references

For a given molecule and electronic configuration, Are the number of contracted atomic orbitals (AOs) different in a restricted open-shell Hartree-Fock (ROHF) calculation from an RHF calculation? ...
1
vote
0answers
62 views

Why must the ground state of hydrogen molecule be gerade?

As we know, the electronic ground state of hydrogen molecule is $\Sigma_g^+$. My question is, how do we know it from the theoretical perspective? The nuclear potential in the hamiltonian of hydrogen ...
2
votes
0answers
25 views

Calculate the Magnitude of Nonadiabatic Coupling

I'm in the great position to be able to calculate nonadiabatic couplings with TDDFT. The result of such a calculation is a displacement for every atom in x, y and z. ...
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votes
2answers
117 views

What is the magnetic quantum number -1 vs +1?

In my book it states the magnetic quantum number “$m_\ell$" is the spatial orientation of the orbital with respect to a standard set of coordinates. I get what $n$ and $\ell$ is, but what does it mean ...
1
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0answers
118 views

Understanding the basics of DFT [closed]

I have recently started doing DFT calculations as part of my PhD. The general approach to DFT in my department appears to be that it is regarded as a tool for doing calculations, which need not be ...
1
vote
0answers
414 views

Why does [Co(NO2)6]4- ion have 3 unpaired electron as opposed to 1?

Why does $\ce{[Co(NO2)6]4-}$ ion have 3 unpaired electron as opposed to 1? The book says that it has 3 unpaired electrons. I thought $\ce{NO2-}$ was a strong field ligand, making $\ce{Co^2+}$ a $\ce{...
3
votes
0answers
179 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 ...
1
vote
1answer
46 views

Small but modest displacements at negligible forces from DFT

When looking at a Gaussian log file today, I noticed the following information in the convergence criterion section after an analytical frequency calculation was performed: ...
9
votes
1answer
215 views

How to identify hydrogen bonds and other non-covalent interactions from structure considerations?

Chemistry is governed by a wide range of interactions, from ionic and covalent bonding, or other types of strong interactions, towards weaker types of bonding, attraction, or repulsion, that typically ...
0
votes
0answers
52 views

Acetone to pyrrole?

Is there any possible route to convert acetone to pyrrole. I tried and took it to pyrazole (after reacting mixed acetone-acetaldehyde aldol product with hydrazine) but I still couldn't find any way to ...
5
votes
1answer
119 views

Fock operator invariance under unitary transformation

I know how to show that the Coulomb operator of the Fock operator is invariant under a unitary transformation of the orbitals, as on page 121 of Szabo and Ostlund, but the indices in my proof for the ...
6
votes
0answers
58 views

Calculating Triplet-Triplet Spin-Orbit Coupling

According to the Q-Chem manual [1], the singlet-triplet spin-orbit couplings between a singlet excited state $I$ and a triplet excited state state $J$ is: $$\langle\Phi_\text{singlet}^I|\hat{\mathrm H}...
8
votes
2answers
212 views

Comparing the Hückel and extended Hückel methods

I'm very confused about the differences between these methods. From my textbook, it states that the Hückel method only takes into account the π bonding interactions, while the extended Hückel method ...
7
votes
1answer
66 views

Angular momentum - Question on Zare's use of the Vector Model to Estimate Probability

My question regards a specific example, although I get the feeling that the answer is simpler than I imagine. In Zare's Angular Momentum: Understanding Spatial Aspects in Chemistry and Physics pgs 53-...
3
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2answers
142 views

When there's a photoelectric effect, do the electrons leave altogether or leave one by one?

When a strong light, or even a laser, hits a metal surface or some molecules, and causes lots of electrons to fly away, do the electrons leave together, or do some of them leave first? If it's the ...
0
votes
0answers
202 views

How to determine the value of charge and multiplicity in Gaussian09?

My molecule of interest is a single negatively charged glucose-based molecule that consists of 30 atoms (9 carbon atoms, 8 oxygen atoms and 13 hydrogen atoms). The total valence electrons are 131 with ...
7
votes
1answer
68 views

D6h symmetry clusters- orbital classification confusion!

I was am simulating high symmetry clusters D6h in several programs such as molcas and gaussian. I understand that the highest symmetry used in these programs is D2h and therefore there would be mixing ...