Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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.

5
votes
0answers
46 views

How to evaluate the expectation value of the Hamiltonian for a three electron Slater determinant?

I'm having trouble with a few concepts dealing with slater determinants and many electron systems. In particular, when dealing with 3 electron systems, I know that $$ \psi_0 = \frac{1}{\sqrt{6}} \det ...
0
votes
0answers
18 views

Why do identical protons not couple with each other in 1H NMR? [duplicate]

So basically I'm wondering why the different protons in ...let's say trans-stilbene do not couple with each other. I'm aware of the fact that they are chemically and magnetically equivalent. They ...
1
vote
2answers
66 views

Concept of enthalpy at a given temperature

In thermodynamics, we always quote a fixed temperature, whenever we mention enthalpy of a reaction. For instance, one can determine the enthalpy of combustion of methane at 25 °C. Now almost all ...
9
votes
1answer
102 views

Strange “far out” orbitals in HF and DFT calculations. Why do they occur?

I calculated some orbitals and I get quite strange results with orbital geometries I wouldn't expect from MO theory. A more experienced colleague just said: "That's always the case with big basis sets,...
1
vote
1answer
75 views

Simulation of Miller-Urey experience [closed]

The Miller-Urey experiment shows that it easy in a primordial atmosphere similar to early earth to generate amino acids,my question: is it theoretically possible to simulate this experience on any ...
5
votes
1answer
69 views

Conformer generation for radicals

Is there an automatic way to generate conformers for radical like systems (spin multiplicity is doublet)? My system for the study is a substituted fullerene with a missing H atom from the functional ...
4
votes
1answer
49 views

Is the neutral di-µ-oxo-bridged manganese dimer with eight ammonia ligands a reasonable structure?

In its guide to model antiferromagnetic couplig, Gaussian uses $\ce{Mn2O2(NH3)8}$ as a model compound.[1] It argues that manganese is in II oxidation high spin state; in other words, in both manganese ...
5
votes
1answer
41 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 $$|sp_{+}\rangle =\frac{1}{\sqrt{2}}\left(\psi_{2s}+\psi_{2p_x}\right)$$ ...
11
votes
1answer
365 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
votes
0answers
72 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?
1
vote
0answers
14 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
votes
1answer
171 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
votes
0answers
23 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) ...
11
votes
2answers
623 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 ...
6
votes
1answer
220 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
votes
1answer
139 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
votes
1answer
131 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 ...
6
votes
1answer
128 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 ...
2
votes
0answers
142 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 ...
11
votes
5answers
551 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
votes
0answers
75 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
votes
1answer
98 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\...
7
votes
3answers
250 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
votes
1answer
357 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
votes
1answer
125 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
votes
1answer
101 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
votes
1answer
128 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
124 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
109 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
1answer
70 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
1answer
93 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
votes
0answers
80 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
210 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
122 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 ...
3
votes
1answer
100 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
174 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
votes
0answers
98 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
votes
1answer
172 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 ...
4
votes
0answers
50 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
145 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
votes
1answer
106 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
79 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}}{...
20
votes
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
44 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
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
votes
1answer
115 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
146 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
61 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
186 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 ...