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

learn more… | top users | synonyms

0
votes
1answer
22 views

Thermodynamics in Solution (solvation) from quantum chemistry

The gist of the question is "how do I get a heat of formation in solution" - but let me give it some context to be more specific. I am interested in calculating the thermodynamic values of molecules ...
5
votes
2answers
73 views

In Chemistry, is it possible to use the old representation of the atom as a nucleus surrounded by orbiting particles?

In Physics, Newton's laws are enough for many applications. Sometimes, relativity must be used instead or complementary, and sometimes, Quantum Mechanics. Is this the same case with Chemistry? Are ...
1
vote
0answers
28 views

How to calculate polarizability?

acks2.in (it's the size of polarizability and density basis sets) control.in - the main input Hardness_matrix Output response_matrix2 response_matrix Sorry to ask a stupid question but I do not ...
1
vote
2answers
37 views

Stability of Bohr Orbits

In class we had been taught that Rutherford's model was unsuccessful because it failed to show that the orbits are stable because the electrons would loose energy because of electromagnetic ...
3
votes
2answers
59 views

Can orbitals in atoms be compressed?

Consider an atom (carbon) with a ground state valence electron configuration of: $$ \underset{3s}{[\uparrow \downarrow]} \underset{3p}{[\uparrow \vert \uparrow \vert \; \; ]} $$ In some molecules ...
4
votes
1answer
117 views

Why is coulomb integral is called local while the corresponding exchange integral is called non-local

In the book "A chemist's guide to DFT" page 27-29 they said that coulomb integral is local interaction term while exchange integral is non-local. I can visualize to some extent that repulsion between ...
4
votes
1answer
85 views

How to explain Born repulsion between ions in gas phase?

In simple terms, the collision of two atoms $\ce{A}$ and $\ce{B}$ will result in ions $\ce{A^+}$ and $\ce{B^-}$ if $$I_a(\ce{A})+E_a(\ce{A})<I_a(\ce{B})+E_a(\ce{B})$$ where $I_a$ and $E_a$ are the ...
0
votes
0answers
27 views

why do we use gaussian type of slater type wave function?

Whenever I read about quantum chemistry, I took it for granted that we need to minimize energy by varying the coefficients/parameters of wave function (aka variational principle). It all makes sense ...
6
votes
1answer
144 views

Why does the local density approximation (LDA) overestimate atomization energy?

I have read numerous times that the local density approximation (LDA) overestimates atomization energy. For example, here is a quote from Dr. Burke's book: LDA typically overbinds molecules by ...
5
votes
2answers
190 views

Typical runtime for CASSCF calculations

I'm doing single point energy calculations of FeS using ORCA. I originally used DFT with a variety of functionals, and these calculations all took less than a minute. Now I'm attempting to run CASSCF ...
1
vote
0answers
16 views

phase differences between conical intersections and light-induced conical intersections?

I've read recently about light-induced conical intersections, a phenomenon where conical intersections can be artificially introduced to molecules, and that this can be observed even in diatomic ...
2
votes
1answer
84 views

Do cyclic oxygen molecules exist and their theoretical stability?

Do oxygen molecules ever become cyclic shapes like carbon and other elements? Why doesn't oxygen form cyclic compounds? How stable would they be if they do exist. Especially more than 3 oxygens. ...
1
vote
0answers
80 views

What are the unsolved problems in quantum chemistry? [closed]

The question is not necessarily limited in quantum, but also computational and theoretical chemistry. Wikipedia listed a few https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_chemistry ...
0
votes
0answers
31 views

Why do ionisations from non-bonding orbitals result in little/no vibrational structure in a photoelectron spectrum?

I understand that ionisations from bonding and antibonding orbitals create vibrational structure from the Franck-Condon principle, resulting in a greater overlap integral between different vibrational ...
15
votes
4answers
208 views

Is it true that all of the physical laws and mathematics needed to describe chemistry are known?

This question comes from a rather famous quote by Paul Dirac which goes like this, The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of ...
1
vote
0answers
36 views

Conical intersection using GAMESS

I would like to know how to locate conical intersection using a GAMESS calculation. I really appreciate an example alone with this since I am a novice I do not have a clear idea among bunch of ...
0
votes
1answer
40 views

Model of London forces and Hydrogen Bonding; the question is carried on forward, partially, as a personal question, from another question [closed]

Does Model of London forces and Hydrogen Bonding are themselves complicated and require multiple sub-models and assisted theories to explain its correctness. I asked this question in the form (...
1
vote
0answers
65 views

How can I get rid of 700 million tons of chlorine gas?

I am conducting a thought experiment in which 40 million tons of hydrogen are supplied through saltwater electrolysis each year, creating 700 million tons of chlorine gas (Cl2) and 800 million tons of ...
3
votes
0answers
21 views

Dexamethasone radius of gyration source

I am looking for a source for the radius of gyration of dexamethasone in aqueous solution. Does anyone know where I could get it from? Even better would be an estimation of the Stokes radius, but the ...
2
votes
1answer
68 views

Is a strontium-fluorine battery the highest voltage battery using pure elements?

Strontium has a very low standard electrode potential and fluorine has a very high one. $$ E_{\ce{F2 + 2e^- -> 2F^-}} = 2.87\mathrm{V} $$ $$ E_{\ce{Sr -> Sr^+ + e^-}} = -4.10\mathrm{V} $$ In ...
1
vote
0answers
104 views

What does Molecular Orbital Theory get wrong?

I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are ...
2
votes
0answers
103 views

Warning during geometry optimization via Gaussian

I have some recurrent problem with my DFT calculations [functional: hseh1pbe; basis:6-31+g(d,p), object: porphyrin derivative]. During the MO setting I get: **** Warning!!: The largest alpha MO ...
7
votes
1answer
94 views

Ballistic behavior of molecules on potential energy surfaces

I came across the following sentence in a publication$^1$: "At this point we have to remember that the computed MEP path [sic] represents the reaction in the absence of any kinetic energy, while in ...
1
vote
0answers
46 views

Are empty orbitals degenerate?

This is apparently a true statement: "In a hydrogen atom, the 2s and 2p subshells have the same energy." Why is this the case? Why would they be of the same energy? Wouldn't the 2p be slightly ...
1
vote
0answers
42 views

Will there ever be a “last element”? [duplicate]

Recently the IUPAC added four new elements to the periodic table and it seems the race for discovering new elements is just getting faster. I was wondering if a "last element" exists; an element with ...
9
votes
1answer
124 views

Why does bond length decrease with increased basis set quality for SCF methods?

From Reviews in Computational Chemistry Vol. 5, for $\ce{H2O}$ using HF, MP2, CCSD, and CCSD(T), all lengths in angstroms: $$ \small \begin{array}{rcccc} \text{Basis} & \text{HF} & \text{MP2}...
0
votes
2answers
51 views

hydroxide ion in water — How does the dynamic change?

It's well known that $\ce{H+}$ ions in water always form $\ce{H3O+}$ ions. Likewise, $\ce{OH-}$ should exist always as $\ce{H3O2-}$ ion. If I am correct at this point, how would the dynamics be ...
1
vote
1answer
85 views

Nuclear zero-point energy and quantum delocalization effects of water molecules

What do we mean by "Nuclear zero-point energy" and "quantum delocalization effects" of water molecules ? It's said that without these effects "water" becomes somewhat unbounded, which is not very well ...
4
votes
1answer
85 views

pka from molecular dynamics

How can one calculate pKa values from first principle calculations, static or dynamics? I have encountered some works using CPMD where people try to find pKa values from -(i) constrained Molecular ...
1
vote
1answer
91 views

Confusion about atomic/molecular orbital terminology

I have come across several different orbital terms: atomic orbitals, natural orbitals, split-localized orbitals, molecule-intrinsic orbitals, quasi-atomic orbitals. I don't know what the difference ...
2
votes
1answer
187 views

What is the Franck-Condon region

What precisely is the Franck-Condon region? My understanding is that the FC region is the "bonding" or "equilibrium" region in a potential energy surface, i.e. near the minimum. But when a transition ...
3
votes
1answer
104 views

Calculating Raman Intensities from Static Raman Activities

Whenever I want to calculate a Raman spectrum, my quantum chemical programs give me Raman scattering activities. As the common Raman spectrum is dependent on the incident wavelength of the used laser ...
6
votes
4answers
4k views

How to calculate the concentration H3O+ in a solution with pH=6.99?

What is the correct way to calculate the concentration $\ce{H3O+}$ in a solution with $\ce{pH}=6.99$? Attempt 1. pH<7, therefore there are only $\ce{H3O+}$ particles in the solution. $[\ce{...
2
votes
0answers
105 views

Why is conjugation not possible if there is not present alternate $\pi$ molecules according to MO theory?

I was reading this page at Chemwiki; there they were discussing conjugated-$\pi$ bond. In order to show that conjugation requires alternate double bonding, they cited the example of 1,3,6-...
4
votes
1answer
62 views

Is it possible to have accumulation of electron density in the inter-nuclear region even if the overlap integral is zero?

Recap: Lately, I've been studying Hückel theory where I learned about the approximation of neglecting all overlaps and making overlap matrix an identity matrix that is $S_{AB}= 0\;_; \;S_{AA}= 1$ or $...
8
votes
2answers
207 views

How can off-diagonal Hamiltonian matrix elements be nonzero when the overlap matrix is diagonal?

While studying about Hückel theory, I got accustomed to the approximation of making the overlap matrix an identity matrix; that is making the off-diagonal elements zero as $S_{AB}= S_{BA}= 0\;;$ this ...
2
votes
1answer
88 views

What is the reason behind neglecting & putting the overlap integrals $0$ in Simple Hückel Theory?

This is an excerpt about Simple Hückel Theory from Elements of Physical Chemistry by Peter Atkins: The first step that Hückel took was to ignore the $\sigma$-bonding framework and focus solely on ...
2
votes
1answer
70 views

What is the significance of Law of Multiple Proportions?

I cannot understand how the Law of Multiple Proportions is significant, or how does it further improve over Law of Conservation of Mass or Law of Definite Proportions. From Wikipedia: If two ...
3
votes
0answers
48 views

Coordinates for molecules except cartesian and zmatrix

Every program in theoretical chemistry that I know uses different ways of writing their input. The same holds true for writing the geometry of the molecules of interest. But they can all be reduced to ...
5
votes
1answer
80 views

Should I include solvation in quantum chemical calculations of a simplified protein-ligand interactions?

I am running a B3LYP-D3/6-31+G geometry optimization of a protein-ligand interaction. Well, not quite, I have simplified the protein to four amino acids surrounding the active site, but I keep the ...
4
votes
2answers
82 views

Do orbital models derive from the solution of the stationary Schrödinger equation?

I know that hydrogen-like atomic orbitals can be seen as a solution of the stationary Schrödinger equation - but what about atoms with more electrons? As I am a pupil and quite uneducated about this ...
3
votes
1answer
125 views

Suggestions for simulating transport through membranes with Molecular Dynamics (MD) and Monte Carlo (MC)?

I'm intending to perform a simulation of the transport of simple ions or water through perm-selective membranes, using Molecular Dynamics (MD) and/or Monte Carlo (MC). I've read around that, in ...
11
votes
1answer
163 views

How important is it that geometry be optimized at a high level of theory?

I've been doing research with a computational chemist for a little while now and in one of our projects we are dealing with a rather large system and because I go to a relatively small university, our ...
5
votes
3answers
73 views

Is there a universal minimum energy state?

I'm imagining this: if everything was separated (even on the nuclear level) and then the parts were recombined so that they were in the lowest possible energy state, do we have any way of knowing what ...
0
votes
1answer
59 views

Double bond equivalent [duplicate]

The formula for degrees of unsaturation is: $$1+ \frac{1}{2} \sum ( n_i (v_i-2))$$ $v_i$ = valency of atom $n_i$ = number of atoms with that valency Could someone explain how the formula works, ...
12
votes
2answers
912 views

Protons and the Heisenberg principle

The Heisenberg uncertainty principle implies that it is not possible to calculate the position and velocity of a body at the same time accurately. Electrons follow this principle since their orbitals ...
1
vote
1answer
113 views

Why do noble gases like Xe and Kr form compounds, despite offending the octet rule?

According to the octet rule noble gases should not form compounds but xenon and krypton form compounds like $\ce{XeF2}$, $\ce{XeF4}$, $\ce{XeOF2}$ and $\ce{KrF2}$ respectively. Why?
10
votes
1answer
197 views

Is it possible that atoms with 120 protons are possible, but that atoms with 119 protons aren't possible?

We currently know that there are atoms with atomic number up to 118 are possible. Is it possible that atoms with 120 protons are possible, but that atoms with 119 protons aren't possible? Or are ...
5
votes
1answer
102 views

Non-bonded orbitals in water

General Chemistry perspective: Looking at the molecular orbitals of water, we can see that the oxygen is $sp^3$ hybridized. Oxygen forms two sigma bonds with hydrogens, and there are two lone pair ...
4
votes
1answer
1k views

Definition of Electronic energy

Using Born-Oppenheimer approximation, we separate the nuclear and electronic parts of the wavefunction, and treat them separately. We include the terms that have direct dependence on the electronic ...