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. 1

    Best Way to Study Quantum Mechanics (QM)

    Hi all, I'm studying Quantum mechanics and there seems to be a lot of conceptual concepts involved. Some of it can be a bit dry and I find myself getting easily distracted. What is the best way to study such a subject? Any feedback would be greatly appreciated. Thanks.
  2. S

    QM uncertainties / probabilities and light

    Greetings, In trying to even ask this question it’s obvious I don’t understand enough about the topic of QM and light. Here is my attempt at my question: If I shine a laser beam at a target or sensor – the electronics every time detects the beam. How do the Schrodinger wave equation...
  3. D

    Exploring the Relationship Between Quantum Mechanics and Macro Events

    Does the notion that a mechanical resonator following the laws of quantum mechanics imply that a similar logic should be used to understand micro and macro events? If there is no border, then why should the logic be different? In other words, if macro events are based on QM, yet they are...
  4. C

    Does QM break down at large scales?

    My friend and I were talking , and my friend said that the laws of QM break down at large scales , But i was thinking that they don’t break down the probabilities just become smaller and it becomes harder to use QM . So what do you guys think
  5. Y

    Conservation of Energy and Mass in QM

    Please tell me how conservation of energy and mass apply in QM. I want to confirm something I read somewhere about QM not conserving energy.
  6. B

    Where does the factor (2π)³ come from in QM Fourier transform convention?

    Homework Statement I'm trying to follow the solution to a homework problem in QM, and I don't fully understand this step. Where does the factor (2\pi)^3 come from? \int...
  7. T

    Linear Algebra Reference for Quantum Mechanics Topics

    hello, is there a reference book in linar algebra that covers topics found in studying quantum mechanics like: vector spaces, operators , matrix , eigenvectors and eigenvalues. i mean not a physics book, i want a mathematics book that talks about these notions and others in an abstarct way...
  8. 8

    Is this an accurate explainantion of QM?

    A buddy of mine asked me to explain QM. I don't by any means tout myself as having any sort of worthwhile knowledge about it so I was a little surprised he asked me. But nevertheless, I explained it to the best of my ability. I'm hoping you guys can tell me if I've royally messed anything up...
  9. S

    How is superfluidity in liquid helium explained by quantum mechanics?

    So I was checking out some cool videos on youtube of helium in superfluid state and I have a question about it. How exactly does this superfluid have both zero and non-zero viscosity at the same time? The evidence seems pretty clear that it exhibits zero viscosity since it forms a Rollin...
  10. K

    Why is superposition prinple a first principle in QM

    I always thought superposition principle is a consequence of the linearity of Schrodinger's equation, but it's not, instead it's a fundamental principle in QM according to some references I read recently. However I did not find any detailed explanation about this, could someone kindly explain to...
  11. P

    2d Wavepacket in relativistic QM

    Dear users, I wonder if there is anybody who can give me a hint on how to handle the following situation: In the 2+1 dimensional Klein-Gordon equation with coordinates (t,x,y), I use as initial condition for \Psi(x,0) a spherically symmetric Gaussian. The relativistic dispersion relation...
  12. H

    QM: Angular Momentum Matrices (Rotating Molecule)

    Homework Statement For l=1 the angular momentum components can be represented by the matrices: \hat{L_{x}} = \hbar \left[ \begin{array}{ccc} 0 & \sqrt{\frac{1}{2}} & 0 \\ \sqrt{\frac{1}{2}} & 0 & \sqrt{\frac{1}{2}} \\ 0 & \sqrt{\frac{1}{2}} & 0 \end{array} \right] \hat{L_{y}} =...
  13. marcus

    Beyond the standard glosses of QM (the realistic clock interpretation)

    We've been watching this development taking shape ever since the tentative Gambini Pullin papers about it in 2006. It represents a fundamentally new way to address the problem of interpreting Quantum Mechanics. One person who worked on this back in 2006, with Gambini and Pullin, used to post...
  14. N

    Electron degeneracy pressure in QM

    In BLACK HOLES AND TIME WARPS by Kip Thorne there is a fascinating discussion about how electron degeneracy pressure fights gravity in dense stars, beginning on page 146. He says the following (abbreviated excerpts) : "..Quantum mechanics insists that when already dense matter is compressed...
  15. DaTario

    Dispersion relation and Photonic Energy and Momentum Equations in QM

    Hi All, In quantum mechanics, related to photon, we have these two equations as valid ones: E = h x f p = h / lambda But we have in vacuum the dispersion relation c = lambda x f. 1) How these relations change when the dispersion relation change ? 2) Is it universally correct to...
  16. N

    What's algebraic approach to QM good for?

    The algebraic formulation of quantum mechanics (and related stuff, like quantum thermodynamics & dynamical systems etc.) via C*-algebras provides a viewpoint based mostly on abstract functional analysis. However, I've yet to see a working application of this approach, i.e. an example of a...
  17. E

    Q: Exploring Metaphysics and Quantum Mechanics

    Hi all I have some questions regarding interpretations of QM. I am not a physicist or mathematician, although I have good general knowledge of science. I'm a philosopher with an interest in the relationships between science, religion and metaphysics. I am trying to link together many...
  18. J

    Retrocausality solves QM problems?

    Retrocausality solves QM problems?? Does retrocausality successfully solve the problems of QM? This recent paper seems to claim it does. http://arxiv.org/abs/1001.5057 Comments, anyone? Also what is the relationship of retrocausality and collapse? TIA Jim Graber
  19. S

    Why do QM particles with l<S vanish in a classical radiation field?

    Off pg 977 MTW Gravitation : Consider a classical radiation field whose associated quantum mechanical particles have integer spin S, and zero rest mass. Resolve that radiation field into spherical harmonics - i.e. into multipole moments. all components with l<S will vanish; in general those...
  20. K

    How necessary is upper level E&M for QM

    Hi I want to take a year long QM squence at my school but the prereqs is a year long sequence in E&M. I was wondering what skills are picked up from E&M that is used in QM. I have taken modern physics which had light exposure to QM and I did not see much drawn from what little E&M i know...
  21. H

    Concept of Superposition in QM?

    Hi I was just wondering if someone could explain superposition in QM? Is it to get the probability of finding a particle in a certain state of a wavefunction that would have both positive and negative probabilities?
  22. E

    How is the density of final states calculated in quantum mechanics?

    Hi. I'm studying the transition rates between a state a and a state b in the continuos level. In the book "Physics of atoms and molecules" by Bransden and Joachain it is said: We have to calculate the density of final states. To do this let the volume V be a cube of side L. We can impose...
  23. J

    Exploring the Mysteries of Quantum Mechanics: A Scientist's Perspective

    hi, I'm new to QM and am a lawyer - only an "armchair" physicist. I would be interested in understanding the "views" of subscribers to this site. Specifically: 1. Do you believe in the story of Schorderker's cat? 2. Is Heisenburg's uncertainty principal still valid? 3. Is the Higgs...
  24. S

    The math required for Relativity and QM ?

    I'm sure this has been posted before, but I did a quick search and couldn't spot anything. I was wondering what textbooks I might be able to self-study in order to get up to speed in mathematics and physics so that I might be able to understand GR, SR, and QM. Currently I'm up to speed on...
  25. MathematicalPhysicist

    Struggling with Integrals in QM? Here's Some Help!

    Questions 3 and 4 in the attachment. The Attempt at a Solution 3. \int d\omega_1 d\omega_2 /|r1-r2|=(2\pi)^2 \int_{0}^{\pi} d\theta_1 \int_{0}^{\pi} d\theta_2 \frac{1}{\sqrt{r_1^2+r_2^2-2r_1r_2cos(\theta_1+\theta_2)} don't know how to proceed from here? for question 4 I got to the...
  26. J

    Thinking conceptually about QM

    I was wondering if the following responses, and their line of reasoning, is correct: The wavefunction is 0 at the nucleus for each stationary state of the hydrogen atom. -> False (thinking in terms of the radial distribution, it only approaches 0) The most probable value of the...
  27. G

    Intuitive Analogy to describe QM

    So I thought of an easy visual analogy to try and grasp the idea of quantum physics, and I wanted to see if this is accurate or if I am off base in my understanding (As I have mentioned, I'm not a physicist.) Picture a point. Draw lines from that point that moving away that are neither...
  28. P

    Quick complex numbers question in QM (probability amplitues)

    [RESOLVED] Quick complex numbers question in QM (probability amplitues) Im a little confused here. I am reading in my textbook about probability amplitudes in Stern Gerlach measurements, and it says this: We find the resulting probabilities for deflection of...
  29. S

    What do you think about Bowman's Essential QM?

    What do you think about Gary Bowman's rather descriptive book "Essential Quantum Mechanics"?
  30. K

    QM Interpretations and the Measurement Problem

    In order to stop hijacking https://www.physicsforums.com/showthread.php?t=361216", I'll restart a discussion here about the role of background assumptions and the conceptual aspects of scientific theories, in particular how the "measurement problem" fits in with quantum mechanics and its...
  31. A

    Can Quantum Mechanics Explain the Uncertainty Principle?

    Hi all, My understanding of the Heisenberg uncertainty principle is this: 1) in order to measure something, we have to hit it with something smaller. 2) in order to know the exact position of something we have to know the exact position of the thing we hit it with. 3) therefore, we cannot...
  32. MathematicalPhysicist

    Find the eigenstates and eigenvalues of the Hamiltonian.

    Homework Statement 1.Consider a one dimensional attraction potential V(x) s.t V(x)<0 for each x. Using the variational principle, show that such a potential has at least one bound state. Hint: use a gaussian in x as a trial functio. 2. A particle with charge e and mass m is confined to...
  33. L

    Exploring Macro vs QM: Bubble Chambers & Uncertainty Principle

    In thinking about QM, I was wondering if anyone could comment on this? Back in prehistoric times, I did research using bubble chambers. In a bubble chamber, you see a number of curved (because of the magnetic field the chamber is imbedded in) tracks corresponding to different particles...
  34. F

    QM book for the GENERAL public

    Hi, I'm looking for a popular science book on quantum mechanics. That is, a book for the general public without heavy math and other scary stuff for us stupid people. I'm not looking for a textbook, so don't recommend Griffiths. I want a minimum level of math (if at all). The level I want...
  35. C

    Can Reproduction and Darwinism Explain the Observer Effect in Quantum Mechanics?

    This is a whacky idea but i thought why not try it out here and see what folks think. This theory is based on accepting an observer-centric interpretation of qm. Many won't agree with that but its important in order to make sense of my argument. So let's pretend that the existence of an...
  36. M

    Discrete or continious spectrum in QM

    Homework Statement Here is the question: how can we know that if we have discrete or continuous spectrum just by looking at the potential graph? Specifically, let`s consider the potential V(x)=-F*x (F:const) . After we solve, we can conclude wavefunctin is airy function, and so both...
  37. F

    QM - Etop of electron distribution of a semiconductor

    Homework Statement I'm trying to find energy level above Ec where electron distribution makes a peak for a nondegenerate semiconductor. For this case we may take GaAs having Eg = 1.42eV at T = 300K. Homework Equations m_e=single isotrophic effective mass or m_0 energy states, g_{c}(E) =...
  38. S

    Can someone give me a geometrical description of the math in QM?

    So I’m trying to visualize what is going on in QM geometrically. More specifically I would like to visualize the time dependent wave equation in 3 dimensions. So let’s start with dimensionality. Normally when I think of a function of some variables, I picture it in a space with the number of...
  39. J

    QM Angular Momentum Commutation Question

    Homework Statement Consider a state | l, m \rangle, an eigenstate of both \hat{L}^{2} and \hat{L}_{z}. Express \hat{L}_{x} in terms of the commutator of \hat{L}_{y} and \hat{L}_{z}, and use the result to demonstrate that \langle \hat{L}_{x} \rangle is zero. Homework Equations [...
  40. G

    Bells inequality be satisfied with equivalent local QM?

    I've just listened to an online lecture where Susskind explained Bell's inequality () Basically he shows that classically A\cap \overline{B}+B\cap \overline{C}\geq A\cap\overline{C} Then he uses spin measurements with 0°, 45°, 90° to the z-axis for A, B, C to measure spins of an electron...
  41. G

    Exploring Cause and Effect in Quantum Mechanics: A Comprehensive Analysis

    It is my understanding that in classical mechanics, cause and effect are universally accepted. Is it the same in QM? Is causality sound in QM?
  42. C

    Light Cones | QM: Understanding the Cone Shapes & Events Outside the Light Cone

    I've always been interested and intrigued by the idea of light cones and how they fit in with other theories in physics. I have a couple of questions about them that I hope make sense are easily clarified: (1) I understand why the shape of the Past Light Cone is a Cone. I don't understand how...
  43. D

    Intro QM: Add a constant to the Potential, Effect on Wave Function?

    Hi, I've got a problem with the following problem. This is 1.8 out of Griffiths QM text, and was previously covered on this forum for another user in https://www.physicsforums.com/showthread.php?t=152775", although that thread doesn't address my problem. 1. Suppose a constant potential...
  44. LarryS

    Can the SR Concept of Reference Frames be Applied to Quantum Mechanics?

    SR "Reference Frames" in QM ? The SR concept of “Reference Frame” cannot be transferred to the micro-world of QM because, due to the HUP, space and time are blurry in that world. Comments? (Thanks in advance).
  45. C

    QM Interpretations: Most Popular & Why?

    The following are the interpretations of QM: Bohmian · CCC · Consistent histories · Copenhagen · Ensemble · Hidden variable theory · Many-worlds · Pondicherry · Quantum logic · Relational · Transactional Which is the most accepted by the theoretical physics community? Obviously all have...
  46. M

    Help with simple numerical ODE problem from QM

    Here's a simple numerical analysis problem that is confusing me. Can someone help me understand what boundary conditions to use here? f''(x) - (x^2 - E_n) * f(x) = 0; Assume f -> 0 as x -> +- inf. This equation comes from Schrodinger's equation for a one dimensional trapping potential...
  47. facenian

    Significance of Phase Factor in Quantum Mechanics Measurements

    If a system is in state |\psi> before a meassurement, then after we find a value a_n the system is in state \frac{P_n|\psi>}{\sqrt{<\psi|P_n|\psi>}} so for instance if |\psi>=c_1|a_1>+c_2|a_2>+c_3|a_3> after we find a_1 the system is in the state |\psi>'=e^{i\theta}|a_1> where \theta= is...
  48. J

    Commutation of operators in QM

    Can somebody please explain the following? Given the measurements of 2 different physical properties are represented by two different operators, why is it possible to know exactly and simultaneously the values for both of the measured quantities only if the operators commute?
  49. F

    Algebraic (ladder) solutions in QM

    When solving the quantum harmonic oscillator often ladder operators (that send energy eigenstates to higher or lower energy eigenstates) are introduced that allow one to algebraically solve the system. Similarly (but with much more difficulty) such operators can be introduced to solve the...
  50. D

    A simple QM experiment analysis question

    Can somebody help analyzing the outcome of the following thought experiment: 1) A simple setup with a photon source, half-silvered mirror and two photon detectors with counters (on each path - A and B); 2) We adjust the setup to get an equal distribution between the counters on paths A and...
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