What is Qm: Definition and 1000 Discussions

The hybrid QM/MM (quantum mechanics/molecular mechanics) approach is a molecular simulation method that combines the strengths of ab initio QM calculations (accuracy) and MM (speed) approaches, thus allowing for the study of chemical processes in solution and in proteins. The QM/MM approach was introduced in the 1976 paper of Warshel and Levitt. They, along with Martin Karplus, won the 2013 Nobel Prize in Chemistry for "the development of multiscale models for complex chemical systems".

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  1. W

    Lagrangian vs Hamiltonian in QFT vs QM

    In QFT, Lagrangian is often mentioned. While in QM, it's the Hamiltonian, Is the direct answer because in QFT "position" of particle is focused on and Lagrangian is mostly about position and coordinate while in QM, potential is mostly focus on and Hamiltonian is mostly about potential and...
  2. StevieTNZ

    How can a particle have infinite positions in quantum mechanics?

    Last night I started reading the section of Brian Cox's "The Quantum Universe" where they discuss how to calculate the probability of a particle being found at a particular position. What he has stated first is if we have an initial position, at any later time we can find the particle at any...
  3. L

    Something's wrong in < Intro to QM >, by Griffiths

    Hi, I am Liu, a undergraduate in mainland China, I post out this idea and want to see how you gentleman think, here it is: Griffiths says, in his book < Introduction to Quantum Mechanics >, if an insect fly in a path perpendicular to the direction in which lightbeam goes, the velocity of...
  4. W

    Understanding QFT vs QM: A Beginner's Guide to the Differences and Similarities

    I'm trying to understand the basics of convensional QFT versus QM. There are too many books about QM in the introductory level for layman but too rare for QFT. But the public needs to be adept about QFT too not just particle-wave duality, entanglement and other attractions in QM. Let's start...
  5. A

    Determination of time-dependent coefficients (QM)

    Hi, I am trying to solve the following equation: \Psi(r,t) = \sum_n C_n(t) e^{-i E_n t} \psi(r) to find the C_n(t)s. The system I am modeling is benzene. I can, by Hückel's method, determine the time-independent solution. The apparently obvious transition from time-independent...
  6. S

    QM focused Documentaries/lectures.

    Hello there. It's fair to say that QM confuses me, a lot. Does anyone recommend any lectures/documentaries that i could watch to further my understanding of QM I'd prefer to watch lectures if i be honest, because i know that documentaries are dumbed down for the public (lets face it they...
  7. N

    [QM] spin and orbital angular momentum: most efficient method?

    Homework Statement Consider a spin-1/2 particle whose state is |\psi \rangle = \psi_+(\textbf r) |+\rangle + \psi_-(\textbf r) |- \rangle. Let \hat{ \textbf S } be the spin observable and \hat{ \textbf L } the orbital angular momentum. We assume that \psi_+(\textbf r) = R(r) \left(...
  8. W

    Simple question about vector spaces and bases in QM

    When reading in Griffiths and on Wikipedia about the vector space formulation of wavefunctions, i am constantly faced with the statement that a vector can be expressed in different bases, but that it's still the same vector. However, I'm having a hard time imagining what it is about a vector...
  9. D

    Is my calculation of carrier density sufficient for all electrons in bulk?

    Hello, I am solving schrodinger and poisson's equation in a self consistent way. I solved schrodinger equation and obtained eigen values which are allowed energy states. now I am calculating electron density with n=sum(psi*psi*f(E)) where f(E)=1/(1+exp((E-Ef)/kT)) fermi function. Now...
  10. H

    The End of Physics and Failure to Integrate QM & Relativity

    I'm not posing this to be a forum troll or to insult the excellent ongoing work in Physics. It's a serious question based on the following: * Most of relativity and QM theory was completed within 20 years by a few dozen scientists. * Their tools were very primitive-not even electric...
  11. romsofia

    QM- Expectation value+Momentum.

    Homework Statement {\Psi (x,t)} = \frac {m \omega}{\pi h_{bar}}^{1/4}e^{- \frac {m \omega}{2h_{bar}}(x^{2}+ \frac {a^2}{2}(1+e^{-2i \omega t}+\frac {ih_{bar}t}{m}-2axe^{-i \omega t})} Problems: Find |ψ(x,t)|2 Compute <x> and <p> Homework Equations {x = \int^\infty_{-\infty} x | \Psi...
  12. S

    QM, GR Unification from a Third Theory

    In a popular book on quantum gravity "Physics meets Philo on the Planck Scale", it is mentioned there are 4 roads to quantum gravity: "1. quantising General Relativity 2. quantising a different classical theory, while still having general relativity emerge as a low- energy (large-distance)...
  13. S

    Momentum Operator question - Proof found in Intro to Griffiths QM

    Hi all I am trying to go through the Griffiths Intro to QM book and I'm afraid I'm already stumped! He determines the momentum operator by beginning with the following equation: <x>=\int_{-\infty}^\infty {x|\psi(x)|^2} He takes the time derivative and manipulates the integral: (I'm...
  14. K

    Time Independant Pertubation Theory - QM

    Homework Statement An electron is confined to a 1 dimensional infinite well 0 \leq x \leq L Use lowest order pertubation theory to determine the shift in the second level due to a pertubation V(x) = -V_0 \frac{x}{L} where Vo is small (0.1eV). Homework Equations [1] E_n \approx...
  15. T

    Particle in a Box: Solving for Acceptable Wave Function with Boundary Conditions

    Homework Statement V(x) = 0 if \frac{-L}{2}<x<\frac{L}{2} and \infty otherwise. Is the wave function \Psi = (2/L)^{1/2} (sin (\pix/L) an acceptable solution to this? Explain Homework Equations H\Psi= E\Psi , normalization: 1 = \int wavefunction^{2}dx The Attempt at a Solution...
  16. K

    QM - Position/Momentum representation problem

    Homework Statement Write down the time independant Schrodinger eqn in the momentum representation for a particle with mass m when the potential is given by V(x) = \frac{1}{2} \gamma x^2 Given that a possible solution is given by \Phi(p) = e^{\frac{-Bp^2}{2}} determine B and the...
  17. DaveC426913

    What causes the polarization effect in optics?

    I've done this experiment several times, and once upon a time I could describe exactly what is shown - but that was a while ago. I need a refresher. Take two polarizing lenses (let's keep it simple - transverse polarization), lens A and B, turn them 90 degrees to each other, they will block all...
  18. M

    Next-Level Quantum Mechanics for Graduate Students

    Hello! I just finished my undergraduate quantum mechanics education and am looking to self-study the next level some time soon (after finals!). We used Gasiorowicz in my class: a god-awful pedagogical text in my opinion. Though I've done pretty well in my course (I'm fairly certain I have...
  19. P

    QM Particles and their space boundaries

    Hi On Wikipedia some author wrote: According to Quantum Mechanics, particles can't inhabit a place smaller than their wavelength. I googled around a little bit but couldn't find any formula which is consistent with this sentence. Anybody knows by which formula this sentence makes sense...
  20. K

    Why is inertial mass not an observable in QM?

    In classical mechanics, I can measure the inertial mass of a particle by measuring force and acceleration: m=F/a. In QM, this equation only holds for expectation values <F> and <a>. Does this lead to the fact that inertial mass is not an observable? Is there a deeper underlying principle which...
  21. fluidistic

    Potential barrier in QM problem with coefficient of transmission

    Homework Statement I'll try to recreate from my memory the problem we've been assigned on a test more than one month ago. They gave the solution but I either misunderstood or miscopied it. An electron with kinetic energy 5 eV goes from a region with potential V_0=6 eV (let's call this region...
  22. E

    Modern theoretical QM research

    Hi, I have a BS in Physics and Math, and am taking a 2 year hiatus to bike to Argentina. I'm currently 3000 miles in, 14000 left. Anyway, I am going to apply to graduate school for admission 2013, and I'm interested in theoretical physics. Primarily, I am interested in the foundations of...
  23. Fredrik

    I want to understand positive operator valued measures in QM

    I want to understand positive operator valued measures in QM, in particular why they are considered "observables". Anyone know a good place to start reading about this? I know some functional analysis and some measure theory, but I haven't made it all the way to the spectral theorem.
  24. fluidistic

    Indistinguishable particles inside infinite potential well, QM

    Homework Statement Assume that inside an infinite potential well there are 2 identical particles that doesn't interactuate between themselves and that have spin 1/2 (for instance electrons). 1)Write down the Schrödinger's equation associated with such a system. Write the eigenfunctions in...
  25. N

    Solving for Constants in Perturbed Simple Harmonic Oscillator with HF Potential

    Homework Statement The potential of a simple harmonic oscillator of HF has the following form \frac{1}{2}kx^2 + bx^3 + cx^4 The first part of the problem involved finding expressions for the first-order energy corrections for the first three states, which I found below. Basically the x3 term...
  26. K

    What Are the Properties of the Commutator in the Dilation Operator?

    Homework Statement Concicer the dilation operator D = \vec{r} * \vec{p} Compute [D,\vec{r} ] and [D, \vec{p}] Homework Equations p = - i * hbar The Attempt at a Solution I think the question is really if [D, \vec{r}] commutes I got this: D = \vec{r} * \vec{p}...
  27. N

    What is the definition of particle(s) in QM?

    What is the definition of "particle(s)" in QM? Hello, It is said that "identical particles are indistinguishable", but I'm beginning to think such a weird sentence is only a result of butchering the word "particle". More concretely: what is actually meant with the word "particle" in the QM...
  28. jfy4

    Is Relational Quantum Mechanics Truly Profound or Simply Confusing?

    Hi, I was looking through Rovelli's Relational QM paper http://arxiv.org/abs/quant-ph/9609002" and Unfortunatly I didn't find the initial set-up of the story compelling. In this set up, there is a system, S, and an observer O. S can be in one of two states |\psi\rangle =\alpha...
  29. Q

    QM - Spin operator conjugate question

    Homework Statement Okay so I've got a question I really need answered first up! If I have a 2x1 matrix for Psi, is Psi* a 1x2 matrix with all the 'i's turned to '-i's? Now onto the actual question - http://imgur.com/3ucb4" - part b only Homework Equations http://imgur.com/bcEm3"...
  30. G

    Can you make a definition of velocity in QM ?

    Can you make a definition of velocity in QM ? I am trying to find a general definition of velocity in QM. QM is totally different world !
  31. R

    Can anyone recommend a text on quantum mechanics based on commutators?

    A footnote to Griffiths reads "In a deep sense all of the mysteries of quantum mechanics can be traced to the fact that position and momentum do not commute. Indeed, some authors take the canonical commutation relation as an axiom of the theory, and use it to derive [itex]p = (\hbar /...
  32. C

    What are some recommended books on interpreting quantum mechanics?

    Hi, I'm looking for a book that presents a pragmatic interpretation of QM that'd be appropriate for someone who's taken one semester of it.
  33. K

    A question for Sakurai's advanced qm

    On page 96, he defined \psi'(x')=S\psi(x), but it seems what he later derived for S only transforms the spinor part not the space-time coordinate of the 4-component wave function, then how is the space-time coordinate primed after acted by S? I'm pretty sure it's not a typo according to what he...
  34. D

    QM: Spin and Angular Momentum.

    Hello All, Right now, I am trying to complete my QM assignment, but I am getting confused about the notation. So, the generalized angular momentum operator is represented as \hat{J} and -j is the quantum # associated with the magnitude and m_{j} is the quantum # associated with the...
  35. C

    QM: Infinite Square Well -a/2 to a/2

    I have read similar threads about this problem but I wasn't able to make progress using them. Homework Statement Consider an infinite square-well potential of width a, but with the coordinate system shifted so that the infinite potential barriers lie at x=\frac{-a}{2} and x=\frac{a}{2}...
  36. jinksys

    Qm - Hermitian Commutator [A,B]†

    I have this review question: If operators A and B are hermitian, prove that their commutator is "anti-hermitian", ie) [A,B]†=-[A,B] What has me confused is the placement of the dagger on the commutator. Why [A,B]† and not [A†,B†]? Also, I am using Griffith's Intro to QM as a text. I have...
  37. bhobba

    Ballentine's Ensemble Interpretation Of QM

    Hi Guys I have recently been reacquainting myself with Ballantines - Quantum Mechanics - A Modern Development. He is pretty big on the Ensemble interpretation, and I must admit I am rather attracted to it as well - none of this collapse of a wave function stuff, many worlds etc. He also makes...
  38. jinksys

    QM- Why is the separation constant +/-l(l+1) ?

    This isn't a HW problem, it's more of a general question. I'm reading Griffith's Intro to QM 3rd Edition. On page 134 he uses separation of variables, of which I'm familiar (but by no means an expert). Could someone explain why the separation constant in equations 4.16 and 4.17 changes sign...
  39. A

    Derivation of the Lorentz force, QM

    Studying in the Heisenberg picture, we have \frac{\text{d}x_i}{\text{d}t}=-\frac{\text{i}}{\hbar}[x_i,H]=\frac{1}{m}\left(p_i-\frac{q}{c}A_i\right) where the last bracket is known as the kinematic momentum \pi . Now, to find \frac{\text{d}^2{\bf{x}}}{\text{d}t^2} I do the following...
  40. J

    Interpretation of Light Beam Intensity in QM

    I am starting to become confused when reading about quantum mechanics and its treatment of photons. I heard somewhere (I can't recall where) that the characteristic intensity of a photon beam was give by I = E/\omega, where \omega is the angular frequency of the beam. However, I can't seem to...
  41. G

    Greiner Mueller's QM II Symmetry

    Is anyone in PF reading the titled book? For me, it is slow work because I have not done physics for a while - decades! I am retired and read some old physics books just for challenge. Mueller gives me that, but also reward. From time to time, I find little stumbling blocks, sometimes a...
  42. V

    Does QM end Free Will/ Determinism debate ?

    Hello :smile: I think my question is clear... Another questions that need to be asked: What about the deterministic interpretations of QM? and Do indeterministic interpretations PROVE that there is really no causality on subatomic level ? Please help me! I am more than a little confused...
  43. TrickyDicky

    Are Vacuum Fluctuations and QM Connected in Feynman Diagrams?

    How are internal lines with no free ends of Feynman diagrams of the perturbative expansion of QFT related to the quantum tunnelling barrier's negative kinetic energy and the HUP of QM? (if they are related at all)
  44. L

    Is Higher Mathematics Crucial in Understanding Quantum Mechanics?

    I'm an undergrad currently taking my first course in QM. We've just about finished our chapters on the mathematical formalism and it has been making me pose a lot of questions. We use a lot of vocabulary/definitions that I suspect come from higher algebra, topology, & functional analysis all...
  45. M

    Phenomena only explicable via QM

    Can someone list some of the phenomena only explicable via QM?
  46. R

    How Does Quantum Mechanics Describe Free-Particle Probability Amplitudes?

    Problem10.1, Introductory QM,Liboff. Homework Statement If \psi (\mathbf{r},t) is a free-particle state and b(\mathbf{k},t) the momentum probability amplitude for this same state, show that \iiint \psi^* \psi d \mathbf{r}=\iiint b^* b d \mathbf{k} Homework Equations \psi_\mathbf{k}...
  47. C

    QM- Stefan's constant vs radiation constant

    QM-- Stefan's constant vs radiation constant Homework Statement (a) Using Planck's formula for the energy density ρ(λ,T), prove that the total energy density is given by ρ(T)=aT4 where a = 8π5k4/(15h3c3). (b) Does this agree with the Stefan-Boltzmann law for the total emissive power? The...
  48. C

    QM: Linear momentum of angular momentum eigenstate

    Homework Statement Find [Lz, Px] and [Lz,Py] and use this to show that \langle l'm'|P_x|lm\rangle = 0 for m' \neq m \pm 1. Homework Equations L_z|lm\rangle = \hbar m |lm\rangle L^2|lm\rangle = \hbar^2 l(l+1)|lm\rangle L_{\pm}|lm\rangle = \hbar \sqrt{l(l+1)-m(m\pm 1)}|l,m\pm 1 \rangle...
  49. T

    How do I move from QM to QFT and beyond?

    Hi, I have now finished University where I took an advanced quantum mechanics module in my final undergraduate year. Having left I was quite surprised to find out that this is infact only a grounding in the subject and I would like to know more. I have covered operators and the Hamiltonian...
  50. B

    How Does Quantum Indeterminacy Relate to the Uncertainty Principle?

    I am an entering college freshman and i have a few questions about the uncertainty principle and hope my misunderstanding can be cleared up. Below is my horrible understanding thus far since my technical abilities are much below necessary for understanding qm, although it is too interesting a...
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