Qm Definition and 1000 Threads

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. Jilang

    I What does simplicial mean in the context of QM?

    What does simplicial and non simplicial mean in the context of quantum mechanics?
  2. N

    I QM objects do not have properties until measured?

    I would like to learn about and clarify the common statement: "QM objects do not have properties until measured". From bhobba in the closed thread: https://www.physicsforums.com/forums/quantum-physics.62/threads/why-the-rush-to-quantum-spookiness.868728/ : "Put a red slip of paper in an...
  3. Smalde

    QM: Time development of the probability of an Eigenvalue

    The problem is actually of an introductory leven in Quantum Mechanics. I am doing a course on atomic and molecular physics and they wanted us to practice again some of the basics. I want to know where I went conceptually wrong because my answer doesn't give a total probability of one, which of...
  4. Einstein's Cat

    Can Quantum Physics Confirm the Correct Interpretation of GHZ and GRW?

    Could physics ever be able to confirm that a certain interpretation of quantum mechanics is correct or is the matter entirely philosophical and thus doomed to the depths of subjectivity?
  5. S

    A Symmetry considerations between observer and observed in QM

    The following is taken from page 101 of Warren Siegel's textbook 'Fields.' Another example is quantum mechanics, where the arbitrariness of the phase of the wave function can be considered a symmetry: Although quantum mechanics can be reformulated in terms of phase-invariant probabilities...
  6. H

    B Is this QM learning pathway sound?

    just for personal enrichment I going to do a second reading of QM. my first was 20+ years ago and found the topic interesting. my education was exclusive to position space and solving problem after problem using SWE with a bazillion different boundary conditions and Hamiltonians, my calculus is...
  7. carllacan

    I Where exactly does QFT differ from QM? (in their formalisms)

    Hi. First off, I'm not sure if this is the right sub to talk about QFT. Apologies if t isn't. I'm halfway through an introductory QFT course and I still don't get what it is. What is different in its formalism that makes it able to tackle problems that quantum mehanics can't deal with? From...
  8. pellman

    I Tensor Product in QM: 1D vs 3D Hilbert Spaces

    A particle in a 1-D Hilbert space would have position basis states ## |x \rangle ## where ## \langle x' | x \rangle = \delta(x'-x) ## A 3-D Hilbert space for one particle might have a basis ## | x,y,z \rangle ## where ##\langle x', y', z' | x,y,z \rangle = \delta(x'-x) \delta (y-y') \delta(z-z')...
  9. J

    I Weinberg LN in QM (Section 3.5): Momentum operator

    Hi everyone, Weinberg uses spatial translation invariance to derive the momentum operator. But the way he does it puzzles me. Here is an excerpt of the book. Equation 3.5.1 is the definition of the unitary operator ##U(x)## for translation invariance: $$U^{-1}(x)XU(x) = X+x,$$ with...
  10. Azruine

    Is this the correct way to solve this QM integral problem?

    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Problem is: If the behavior of ψ( r,t ) as r->inf is dominated by r-n, what values can n assume if the integral ∫A(ψ*∇ψ-ψ∇ψ*)⋅nda taken over the surface at infinity is to vanish. I considered ψ...
  11. N

    (Hard QM) Consider the double well potential with delta func

    Homework Statement Consider the double well potential with two wells separated by delta function in middle. V(x) = V0 > 0 for x<-a and x>a 0 for -a<x<0 and 0<x<a αδ for x =0 1. Find Bound state energies 2. Find odd solns and their eigenvalue equation. Give solns in...
  12. T

    I Why is QM (almost) local, with one exception?

    ... except one weak exception of the entanglement? Assuming that unitary QM (aka MWI) is a-local (c) Demystifier, and the ultimate reality is the Hamiltonian, somehow mapped into our 3D space, it is more surprising that the nature we observe is local. If would be more logical to expect multiple...
  13. S

    I Why is the KE operator negative in QM?

    In the Hamilonian for an H2+, the kinetic energy of the electron (KE of nucleus ignored due to born-oppenheimer approximation) has a negative sign in front of it. I understand the signs for the potential energy operators but not for the KE apart from the strictly mathematical point of view. Can...
  14. J

    I Weinberg Lectures in QM (2013 Ed.), Equation 7.10.15

    Hello everyone, I don't get how the second-order derivative ##\partial^2 S/\partial x_i \partial x_j## of the phase S arrives here. If one performs a power series expansion of the Hamiltonian around ##\nabla S##, then I do get where the first term ##A## comes from, but then adding higher-order...
  15. F

    I Does the probability collapse theory of QM imply backward in time

    In the thought experiment known as Schrodinger's cat a cat is placed in a sealed box, and its life or death is tied to an uncertain quantum event such as radioactive decay. If the radioactive particle decays, the cat dies. If not, nothing happens. According to probability collapse QM, as...
  16. J

    I Weinberg Lectures on QM (2013 ed.), Equation 6.5.5

    Hi everyone, I'm a bit puzzled by the derivation of this formula, in particular since the definition of the "overbar" notation is a bit fuzzy (see Formula 6.4.1). Does anyone have a more formal definition of the correlation function in this setting (I know what a CF is, in general)? In this...
  17. H

    B Is Solving Schrodinger's Equation the Best Way to Approach Quantum Mechanics?

    Wondering what educators / researchers in the field think is the most important and logical flow of topics. The way I was instructed was solving Schrodinger's wave equation, SWE, for every possible problem that could be solved with a pen and paper in say 2 or 3 pages max. The problems were...
  18. F

    I Can subspaces be used to determine probabilities in quantum mechanics?

    Suppose we have an observable with a certain number of eigenstates. We would normalize all these possibilities to 1 in order to give each eigenstate an appropriate probability of being measured. Can we then only consider the data of many measurements for only a subset of those eigenstates and...
  19. durant35

    I Difference between atomic behavior in QM and classical physics

    I believe this question will be a no-brainer for many members. I want to conceptually grasp how do atoms (and molecules) behave using QM in contrast to the classical physics model. Now there's uncertainty principle and atoms don't have a fixed position and momentum, and that's fine, I know it's...
  20. T

    Moving charge magnetism explanation

    I'm from a country in Africa where I didn't have the privilege of learning the Standard Model, so I'll try and rephrase my question here: https://www.physicsforums.com/threads/why-is-the-electromagnetic-field-not-a-charge-field.857927/#post-5384284 In a way where my ignorance doesn't outrage...
  21. F

    I Exploring the Connection Between Quantum Mechanics and Quantum Field Theory

    << Moderator note: Split from https://www.physicsforums.com/threads/why-do-we-need-quantum-mechanics-so-much.859210/ >> What about particle physics? It bases on QFT therefore on QM.Is that right?
  22. J

    Weinberg Lectures on QM (2013 ed.), Equation 3.6.18

    Hi, I don't get how one goes from 3.6.17 to 3.6.18 on Page 80 (Galilean invariance) regarding the zeroing of boost generator commutators. I do get that this is a special case of the Lorentz invariance (which I understand), but this particular step eludes me. Thanks for your help.
  23. DaTario

    A Amplitudes of probability in Mathematics before QM

    Hi All, Was there any use of the concept of amplitudes of probability before their use in quantum mechanics? In connection to this question, who invented or was the first to use this resource? Best wishes, DaTario
  24. sa1988

    QM: "What are the possible results of measuring Operator A?"

    Homework Statement Homework EquationsThe Attempt at a Solution I'm fine with parts a) and b) However I don't understand what part c) is asking me to do. How do I 'measure' an operator? There are only two things I can think to do: 1. Find the expectation values of A for <Φ1|A|Φ1> and...
  25. Zafa Pi

    I Are the 2nd and 3rd axioms of QM incompatible?

    Nielsen & Chuang list three axioms for QM. I paraphrase them as follows: 1. States are unit vectors. 2. The evolution of a state is unitary and given by the Schrodinger equation. 3. The measurement of a state yields a value from a probability distribution. The state just before the...
  26. entropy1

    Can QM interpretations be reconciled?

    Is there already consesus in the (mainstream) scientific world that the various interpretations of QM can't and won't be reconsiled? (unless, perhaps, they find a completely different mathematical framework?) And what is the cause of that? :smile:
  27. G

    Why QM (or rather, nature) is weird, in lay terms.

    If I show you three face down cards (normal playing cards, so can be either a red or black suit), and tell you that no matter which two you pick, they would be different colors, I have no doubt you'll say I'm being weird (to put it mildly). Well, nature does pretty much the same thing. This is...
  28. S

    Exploring the Wave-Particle Duality of Electrons in CRT Monitors

    How do CRTs work well and electrons can be sent to exact location on screen in CRT monitors if electrons can behave like wave? Is there something in old TVs (for example measurement device) along the road that electron travels to avoid behave like wave?
  29. durant35

    Understanding the Measurement and Interaction of Electrons: A Beginner's Guide

    Hi guys, I'm a newbie in quantum physics and it has blown my mind so far. I feel a bit confused because it seems very unintuitive, but I'm ready to learn more and I need some help on this forum. I have few questions. 1) I red that the wavefunction of an electron is spread all over the universe...
  30. J

    Axiom of QM is Observation... but

    It is said the axiom of QM is observation.. but if observation is secondary effect of more primary dynamics that don't involve observations.. could it still be called QM, and what's it supposed to be called? In other words. Say QM is emergent from a deeper reality.. and we are to study the...
  31. entropy1

    QM interpretations defined as eigenvalues?

    And back again with a strange/odd layman question: Actually, this remark of Jilang is a perfect illustration of what I am wondering about: It seems as if realism and locality are behaving like two eigenstates in a space of interpretations of QM. Most people seem to suggest that you must have...
  32. G

    Zettili QM Problem on Trace of an Operator

    Homework Statement In Zettili's QM textbook, we are asked to find the trace of an operator |\psi><\chi| . Where the kets |\psi> and |\chi> are equal to some (irrelevant, for the purposes of this question) linear combinations of two orthonormal basis kets. Homework Equations...
  33. thenewmans

    QM interpretation with something moving backward in time?

    It’s easy enough to find the interpretations that break counterfactual definiteness in order to maintain locality. But I’m not sure how to find the ones with something (perhaps information) moving backwards through time. The reason I ask is I’ve always had trouble with the idea that a...
  34. J

    What kind of differential equations one must know for QM?

    I will be taking a first course on Quantum Mechanics and just wanted to know what kind of ordinary differential equations must i know before going into the course. Thank you!
  35. H

    Unifying GR and QM - Reasons for Believing in Unity

    I am new to this forum and relatively new to learning physics itself. I have a very basic question. What is the motivation for looking for a unified theory of quantum gravity? Other than the aesthetics of having a unified understanding of reality, do we have any physical or mathematical reason...
  36. A

    Can QM be described by Markov chain theory?

    Can we describe the intensity of spectral lines using Markov theory? No matter what is the initial state vector of the system, the final state will be reduced to a stationary vector whose elements represent the intensity of the spectral lines.
  37. J

    Integrating Gaussian Distribution (QM)

    Homework Statement I am struggling with one of the end of chapter questions in my QM textbook (see attachment as I don't know how to show calculus on PF). It has thrown me because the chapter introduces some of the key principles in QM by talking about probability but then it randomly chucks in...
  38. V

    Recovering QM from QFT: David Tong Notes

    Reading through David Tong lecture notes on QFT.On pages 43-44, he recovers QM from QFT. See below link: [QFT notes by Tong][1] [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdfFirst the momentum and position operators are defined in terms of "integrals" and after considering states that...
  39. T

    Exploring Quantum Mechanics Through Experiments

    Hi. I hope this is the rightplace to ask this. Sorry if it isn't. I'm looking for a resource (book, web page, etc) and I don't know if it exists so I'll describe it. I want something that lists actual experiments that complement or back up the topics listed in a typical QM textbook, so I can...
  40. A

    A Is the Spectral Gap Problem in Quantum Mechanics Undecidable?

    There is a group of researchers that say that there is a problem in QM (the spectral gap problem) that is undecidable in generalhttp://www.nature.com/news/paradox-at-the-heart-of-mathematics-makes-physics-problem-unanswerable-1.18983?WT.mc_id=TWT_NatureNews...
  41. B

    Can the "many-worlds" in QM inter-communicate?

    My layman understanding of the many-worlds interpretation of QM is that if a person had to make a decision between A and B and he chose A, then the universe splits into 2 branches, one where A was chosen and another where B was chosen. First of all, is this correct? Secondly, regarding the...
  42. entropy1

    Self-Study Quantum Mechanics: Can I Do It w/ Poor Math Skills?

    Whole my life I have been interested in Quantum Physics. I have a bachelor degree in IT. I did not finish my studies. I have always been quite sloppy in studying in school. As a result, my mathematics skills are terrible. I often get pointed out on this forum that I better start with the...
  43. kini.Amith

    Quantum Which Intermediate Quantum Mechanics Textbook Covers Specific Topics Best?

    I just finished a first course in quantum mechanics based on Griffiths' textbook. I have to take an advanced qm course next sem which starts in Feb. I was planning to do some reading by then mostly on some specific topics like mathematical formalism of QM, spin and angular momenta, and identical...
  44. A

    Why do some QM books start with Polarization?

    I'm an undergraduate in Physics and just cracked open some QM books, some seem to start with polarization and a strange way of introducing operators, while others start with the wavefunction and then go into harmonic oscilators, then the hydrogen atom etc.. What does polarization have to do...
  45. N

    Example of Susy QM and its partition function

    I am reading an article by Tachikawa on the Nekrasov partition function ("A review on instanton counting and W-algebras"). The article is meant to be pedagogical but I have some trouble with what is supposed to be "baby" examples. The first one involves susy QM on \mathbb{C}^2 . He says the...
  46. entropy1

    I Causality in Quantum Mechanics: Exploring Meaning & Timing

    I am wondering about the meaning of 'preservation of causality' in quantum mechanics. Is there causality in QM? And does it act back in time? I have some ideas of myself, but want to learn about the general accepted view first.
  47. W

    Quantum QM books in order from easy to intermediate to hard

    So far I got: Griffiths => easy, ok it's not so easy,not really what I ment, but it covers the basics without going much into the real deeper algebraïc stuff, emphasis on calculus, that's what I mean with easy, I mean a decent, solid book for undergraduate level. But suppose I tackle Griffiths...
  48. N

    Depending on interpretation of QM, can Hilbert space be....

    Depending on interpretation of QM, can hilbert space be considered just as real as space time? In MWI the wave function is real, but still lies in hilbert space, so would hilbert space be considered a real space according to this interpretation?
  49. Z

    QM: commutator 2D particle magnetic field

    Homework Statement I was reading this textbook: https://books.google.com/books?id=sHJRFHz1rYsC&lpg=PA317&ots=RpEYQhecTX&dq=orbit%20center%20operators&pg=PA310#v=onepage&q=orbit%20center%20operators&f=false Homework Equations In the equation of the page (unlabeled), we have $$...
  50. F

    An Easy QM normalisation question

    Homework Statement Given a wave function psi, \psi (x) = A \sqrt{|x|} e^{- \beta x^2} where \beta is a constant (take the positive square root) . Normalise the wave function and hence find A. Homework Equations ? The Attempt at a Solution This is my first attempt at a problem like this, and...
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