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Questions tagged [theoretical-chemistry]

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

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36 views

What determines the coefficients in the wavefunction of a hybrid orbital?

I was studying the hybridization of carbon atoms and I came to figure out that the $sp$ hybridization wavefunction is given as $$|sp_{+}\rangle =\frac{1}{\sqrt{2}}\left(\psi_{2s}+\psi_{2p_x}\right)$$ ...
10
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1answer
350 views

Is phosphorine (C₅H₅P) aromatic?

Phophorine seems aromatic as it has 6 conjugated electrons. But the answer given is that it is not. This seems odd since pyridine has a similar structure and is also aromatic. Thus I ask is ...
7
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0answers
66 views

Is the 9H-fluoren-9-ylium cation aromatic?

In this cation, there are two individual benzene rings which are aromatic. But in the entire π system, there are 12 π-electrons which should make it antiromatic. Which is correct?
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0answers
12 views

How does mesh size of Barium Chloride affect sulfate results via turbidimetric method?

I am analyzing sulfate using Standard Methods 4500 $\ce{SO4 ^2-}$ E (turbidimetric method) using $\ce{BaCl2}$ and measuring %T at 420 nm. In the past, we used an appropriate 20-30 mesh size for the $\...
7
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1answer
136 views

Can Le Chatelier's principle be derived?

Le Chatelier's principle says : If a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as ...
2
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0answers
19 views

GVP(PP) orbital energies?

Hartree-Fock orbital energies can be interpreted physically, e.g. in Koopmans theorem, while one cannot assign active MCSCF orbitals to energies in general. What about GVB, say for example GVB(PP) ...
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2answers
611 views

Is the notion of orbitals different in theoretical chemistry?

I am just an average chemistry student that is currently exposed to theoretical chemistry. And I am seeing very puzzling things, such as 1p, 2f, 3g orbitals. In my attempt to reconcile this, I was ...
7
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1answer
181 views

Hartree product and the Slater determinant

I am very new to theoretical chemistry so my question may sound very basic to most. It is known that the Hartree product does not satisfy the antisymmetry principle for electrons while the Slater ...
4
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1answer
95 views

How to explain the excited states in the dinitrogen cation?

The $\ce{N2^+}$ molecule has the molecular orbitals $$\ce{\sigma(1s)^2\; \sigma^*(1s)^2\; \sigma(2s)^2\; \sigma^*(2s)^2\; \pi(2p_x)^2 \; \pi(2p_y)^2 \; \sigma(2p_z)^1,}$$ which can be seen on the ...
6
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1answer
71 views

What exactly is meant by 'multi-configurational' and 'multireference'?

Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet. I would like to know what exact property of the wave function these terms refer to. It would ...
5
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1answer
103 views

Is the Lowdin Orthogonalization used in diagonalizing the atomic orbitals really a basis change?

This is kind of more on the mathematics side of quantum chemistry, but I can't quite figure out why the Lowdin Orthogonalization is called a basis set change. I get how it works from the perspective ...
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0answers
128 views

What studies show that d orbitals don't play an important role in covalent bonding? (> period 2 elements) [closed]

Take a look at all the following molecules: $\ce{SO3^{2-}}$, $\ce{SO2}$, $\ce{SO4^2-}$, $\ce{PF5}$ and many more you can add. In all these there is a controversy of one of these two types: Is formal ...
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5answers
515 views

Visualizing atoms in XYZ file with colors representing partial charges

I have a file containing the atomic coordinates (in XYZ format) of a structure, and I also have a list of the partial atomic charges for each atom. I'd like to create an image of the structure where ...
4
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0answers
72 views

Number of configurations/states for electrons in orbitals

I'm performing complete active space calculations for the prediction of X-ray absorption L edge spectra. This involves an excitation from 2p orbitals to the 3d orbitals of the metal. For the ...
5
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1answer
86 views

How are the LCAO ansatz and Slater determinant ansatz for the wavefunction connected?

Say we have a system of 2 electrons, an ansatz for the wavefunction is then a Slater determinant consisting of two orbitals. $$ \left|\Psi_{Slater}\right>= \frac{1}{\sqrt{2!}}\left(\left|\phi_1\...
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3answers
233 views

What is Nesbet's theorem?

In a talk by Frank Neese there was a slide with title "Nesbet's theorem" and an equation indicating something like "if your wavefunction has the double excitation amplitudes correct, then you can get ...
6
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1answer
245 views

Why can't NH5 form?

In ammonia a nitrogen atom forms 3 covalent bonds with hydrogen atoms and has one lone pair which can be used in a dative covalent bond with another hydrogen atom to form an ammonium ion. Instead of ...
5
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1answer
120 views

Detail from Integral Evaluation in Szabo (Appendix)

This is a step from Appendix A in Szabo and Ostlund's Quantum Chemistry. In short, the question is how to get from \ref{A.20} to \ref{A.21}. We can factor out some constants $K$ in the expressions to ...
3
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1answer
73 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 ...
6
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1answer
126 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 ...
3
votes
1answer
95 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 ...
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0answers
80 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
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1answer
59 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
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1answer
78 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 ...
5
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0answers
74 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
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1answer
206 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
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1answer
116 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
2answers
129 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
95 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
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1answer
147 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
95 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 ...
7
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1answer
147 views

Why is Coupled Cluster not variational?

It has been noted in several sources (e.g. J. Romero et al. Strategies for quantum computing molecular energies using the unitary coupled cluster ansatz. arXiv:1701.02691 [quant-ph]) that one of the ...
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0answers
47 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 ...
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2answers
141 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. ...
5
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1answer
94 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
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1answer
76 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}}{...
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5answers
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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
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1answer
41 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 ...
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1answer
54 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
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1answer
109 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
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1answer
133 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 ...
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0answers
59 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
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1answer
146 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 ...
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0answers
98 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 ...
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1answer
25 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.
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1answer
71 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
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1answer
97 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 ...
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1answer
969 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 ...
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0answers
40 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 ...
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0answers
62 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>, |...