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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21 views

What makes determining entropy changes theoretically so hard compared to enthalpy?

It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
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0answers
39 views

Is there a reliable chemical theory that predicts pKa based on structure? [on hold]

Obviously, there are general stability arguments that can be made to estimate relative pKas, such as evaluating the stability of the conjugate base of an acid, or thinking about how polarized the bond ...
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0answers
58 views

Do “random” “2D” polymers constrained to a box actually form “space filling” curves?

Background: The self avoiding walk model is (allegedly) a reasonable model for polymers, at least with respect to certain "universal" (i.e. lattice independent) constants -- see section 1.1. in this ...
4
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1answer
93 views

How to calculate S² value of a broken-symmetry wave function?

$S$ represents spin, signifies the number of unpaired electrons in the system. For example, if the number of unpaired electrons is $1$, then $S=1/2$. $S^2$ is calculated as $S(S+1)$. From what I have ...
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0answers
11 views

difference between configuration state functions and configuration interaction? [closed]

What is the difference between configuration state functions and configuration interaction? Please explain the difference and thank you.
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0answers
27 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 ...
3
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0answers
30 views

All possible determinants forms a complete basis [closed]

How do we mathematically prove that the set of all possible excited determinants forms a complete basis for expanding the wavefunction (as in full configuration interaction)?
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0answers
31 views

Products of Redox Equations [duplicate]

I was just wondering how would you know the products of a redox equation, or if that could even be derived from just knowing the reactants. For example when Potassium permanganate reacts with a ...
1
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0answers
43 views

Why does a 4-bonded nitrogen atom appears negatively charged after an optimization?

I have a molecule bearing a quaternary ammonium and I optimized it using M062X (also Hartree-Fock) using basis sets 6-31G(d,p) and 6-31G+(d,p) respectively. After optimization, I expect that the ...
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0answers
26 views

Is there an easy explanation of MP2 calculation [closed]

for my master thesis I calculated some ring strains with MP2 calculation. It was not hard to calculate these things but I'm struggling to understand what the theory is about. Now I'm defending my work ...
4
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0answers
55 views

Variational method: Hydrogen atom ground state in STO-3G basis expansion

I computed numerically the ground state energy of hydrogen by variational procedure, firstly expanding the wave function over the s-wave basis set STO-3G $$\psi(r)=\sum_{i=0}^3 C_{i} e^{-\alpha_ir^2}$$...
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0answers
46 views

Is there a mathematical basis behind the HSAB theory?

I learnt about the HSAB concept about a year ago, and found that it helped rationalise many of the solubilities/reactivities that “pure” Lewis acid/base theory could not. Is there a mathematical ...
2
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0answers
52 views

Antisymmetrized Product of Strongly Orthogonal Geminal (APSG) method

To get a deeper understanding of electronic structure methods, like Hartree-Fock, MP2, and Coupled Cluster method, there are many small programs (codes) available in Python, C++ or Fortran, for ...
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0answers
80 views

Hartree Fock for an atom (Beryllium)?

I'm working on solution of Hartree-Fock equation for Beryllium in configuration $1s^2 2s^2$, expanding the orbital with STO-3G basis $$\phi_k=\sum_{p=0}^3 C_{kp}e^{-\alpha_{kp}r^2}$$ I know the values ...
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2answers
49 views

Spectroscopic constants - explanation

I'm a beginner in spectroscopy and I have troubles understanding papers describing values of spectroscopic constants. This paper, for example, describes constants $T_e, R_e, \omega_e, \omega_ex_e, B_e,...
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0answers
88 views

Why does the exchange interaction in Hartree-Fock theory lower the total energy?

In Hartree-Fock theory, the expectation value of the total energy can be written as $$E = \langle\Psi| H |\Psi\rangle = \sum_{a} \langle a| h |a \rangle + \frac{1}{2}\sum_{ab} \big( [aa |bb] - [ab|ba]...
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1answer
48 views

Charge, multiplicity, level of theory and coordination coordinates of gold compleaxes [closed]

Good day, I need to learn a trick in answering the following questions: How to come up with cartesian coordinates of a complex say, [Au(CH2)2(S)PPh2]2, to be ran on Gaussian? The charge and its ...
1
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1answer
33 views

Number of stable molecules of artificial molecule XY4 [closed]

Suppose element X has $5$ stable isotopes, and element Y has $6$ stable isotopes. Find the number of natural molecules, knowing that an X has a charge of $+4$ and Y has a charge of $-1$. What I tried ...
7
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1answer
168 views

What is the difference between a configuration state function and a Slater determinant?

I do not understand the distinction between configuration state functions and Slater determinants. Is not every Slater determinant a CSF ? Please explain the difference and the use/need of ...
1
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2answers
78 views

Dissociation degree hell, Ostwald's law of dilution

Greetings dear chemists! I got a nice exam problem on wich i am stuck for hours, well maybe the problem is with me. I know the problem can be solved. So here we go. We got a weak monoprotic acid, we ...
2
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0answers
59 views

What is the ANO-RCC basis set and how does it work?

On the basis set exchange, the ANO-RCC basis looks huge, even for hydrogen: ...
2
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1answer
35 views

A theoretical framework for the formalization of chemical interaction and configuration based properties

Is there an attempt in any area of chemistry to formalize the way a molecule's properties change based on its configuration and the resulting changes in interaction with other molecules. For example, ...
1
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0answers
45 views

GAMESS Gaussian qst2 method alternative

I've heard about the qst2 Gaussian method but I don't have any licence so I use the free GAMESS-US. Nevertheless, I didn't find any Gamess method which allows a TS finding like qst2. Have you heard ...
5
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2answers
72 views

Intuition for why catalyst affects both forward and reverse reactions equally?

I have looked for quite a long time on the site however all the answers involve mathematical explanations involving Arrhenius's equation. I would really like an intuitive reason (or one using ...
9
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0answers
177 views

What is the largest coupled cluster calculation that has ever been done (as of March 2019)?

I am doing some research and trying to make sure the numbers I am reporting are accurate. I found that the paper by Sylvetsky et. al. says:[1] We were, however, able to complete a ...
8
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1answer
247 views

Pauli exclusion principle

Pauli exclusion principle states that 'No two electron can exist in same state' or 'No two electron can have same set of quantum numbers'. But in reality there is no state of electron. The state is ...
4
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2answers
153 views

Ab initio calculations

Why are quantum mechanical calculations so computationally expensive? What exactly is the process that makes it so time taking? What I mean is, one can perform classical dynamics simulations on a ...
2
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0answers
93 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 ...
6
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0answers
98 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
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0answers
23 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
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2answers
82 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 ...
10
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2answers
143 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
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1answer
95 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
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1answer
106 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
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1answer
56 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
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1answer
47 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
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1answer
403 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
78 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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27 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
312 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
25 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
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2answers
642 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
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1answer
327 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
616 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 ...
9
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1answer
460 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 ...
7
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1answer
216 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
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0answers
163 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
754 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
81 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
117 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\...