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. P

    Mathematica QM Animation of delta potential

    I'm trying to use the Animate function on Mathematica to show a gaussian wave packet passing through a delta potential. I'm quite new to Mathematica and this is by far the hardest thing I've had to do so please bear with me. I effectively want to solve the integral: ## \phi_k(x) = \left\{...
  2. E

    B Does Dirac's Conclusion on Electron Velocity Still Align With Modern Physics?

    In Dirac’s QM book, Revised Fourth Edition (~1967), Chapter XI Relativistic Theory of the Electron, page 262, he reaches the conclusion that “…a measurement of a component of the velocity of a free electron is certain to lead to the result ±c.” Is this still the current thinking? (He is talking...
  3. bananabandana

    Why are unperturbed states valid basis for perturbed system?

    Homework Statement So we have a two state system, with unperturbed eigenstates ## |\phi_{1}\rangle##, ## |\phi_{2}\rangle ##, and Hamiltonian ## \mathbf{\hat{H_{0}}}## - i.e ##\mathbf{\hat{H_{0}}}|\phi_{1}\rangle = E_{1}|\phi_{1}\rangle## We shine some z-polarized light on the system. This...
  4. N

    I Non linear interpretations of QM and string theory?

    Are interpretations that are non linear with respect to the wave function (eg. GRW, transactional) compatible with string theory?
  5. N

    I Do nonlocal QM interpretations violate relativity?

    Do interpretations of QM that are non local (Bohmian, collapse, transactional) violate relativity since they allow for faster than light travel?
  6. N

    I Interpretation of QM that psiontic, local, and has collapse?

    Is there any interpretation of QM that is Psi-ontic, local, and has wave function collapse?
  7. A

    Canonical commutation relation, from QM to QFT.

    Homework Statement This is a system of n coupled harmonic oscillators in 1 dimension. [/B] Since the distance between neighboring oscillators is ## \Delta x ## one can characterize the oscillators equally well by ## q(x,t) ## instead of ## q_j(t) ##. Then ## q_{j \pm 1} ## should be replaced...
  8. Auto-Didact

    A Experiments probing the macroscopic limits of QM

    This thread is to serve as both a compilation and ground of discussion of key experiments, both historical and planned, which attempt to probe possible macroscopic limits of QM, taking into account e.g. some particular gravitational/optical/mechanical/superconducting/etc aspect and/or...
  9. Auto-Didact

    A Weinberg: detecting changes to QM using atomic clocks

    Steven Weinberg has lately been critical of QM. He now also has a technical paper out called 'Lindblad Decoherence in Atomic Clocks', available on arxiv. Here is the abstract: It's a short paper (6 pgs of text), arguing for objective collapse (a la GRW/Diosi-Penrose/etc) instead of...
  10. S

    I Occam's razor and probability in QM

    I am a laymen of sorts in both physics and philosophy. I embarked on a trip to acquaint myself (gently at first) with contemporary physics. My question is foundational and therefore probably philosophical. If it is off-topic, please kindly point me to any place where such discussions may be...
  11. AMuonParticle

    Other I need recommendations for an interesting Senior Project.

    I'm currently a junior in high school, and I'm really interested in physics, especially particle physics and quantum mechanics. At my school, everybody needs to do a big senior project to graduate, involving multiple papers and about 30 hours of work, usually to build something or organize a...
  12. ChrisisC

    What Subfield of Physics has the *most* QM involved in it?

    i'm assuming is particle physics... but could it be anything else? I am curious to know because i am trying to lay out a path for my educational career and i definitely know physics is my path. However, i don't know exactly was field of physics i want to specialize in. I think QM is the most...
  13. ChrisisC

    B How close are we to uniting gravity and QM

    How much progression has physics made in uniting QM and gravity? Have we made any steps to unite them? have we discovered anything remotely related to it?
  14. F

    B QM: Quantization Dependent on Spatial Confinement?

    Hello Forum, Some observables in quantum mechanics are always quantized, i.e. come in discrete separate values, while other observables, like energy or momentum, can be quantized or not depending on the spatial confinement of the system. Is that correct? Is spatial confinement of the physical...
  15. Andreas C

    I What do you think of the alternate interpretations of QM?

    The standard interpretation of QM advocates that there are no "hidden variables", and that the universe is not deterministic. Sure, maybe Bell's work supports that view, but there exist some deterministic theories of quantum mechanics (like the De Broglie-Bohm theory) or other stuff, like the...
  16. N

    I Is there any way to get a geometrical description of QM

    This is maybe one of my greatest gripes with QM, I have never seen a geometrical description of it. What I mean by geometrical, is a description of the given object in the 3D world we live in, not a description in Hilbert Space, is such a description even possible in principle? I've been...
  17. F

    I State Representation in QM and Vector Spaces

    Hello Forum, The state of a quantum system is indicated by##\Psi## in Dirac notation. Every observable (position, momentum, energy, angular momentum, spin, etc.) corresponds to a linear operator that acts on ##\Psi##.Every operator has its own set of eigenstates which form an orthonormal basis...
  18. .Scott

    I QM Ignorance is Bliss: Exploring the Limits of MWI

    The purpose of this post is to describe what may be a useful way of viewing some aspects of QM. It probably doesn't rise to the point of a testable theory. Over a year ago I argued against the Many Worlds Interpretation based on quantum conservation of information. My argument was this: Any...
  19. J

    When did QM become standard Curriculum

    Hi all, Given that usually the most recent subject taught in a standard undergraduate curriculum is quantum mechanics, which was developed in the first half of the twentieth century, when did it become a standard part of the curriculum for undergraduates?
  20. collinsmark

    B QM Humor: "Heisenberg Uncertainty Principle

    I was going to post this in the general discussion sub-forum (like the science jokes thread), but I think it's actually technical enough to be more appropriate here. Today's SMBC (source: http://www.smbc-comics.com/comic/the-talk-4)...
  21. K

    Can a Student with Struggles in Advanced Topics Succeed in Physics?

    I'm a 20 years old student in the first year course to get a bachelor degree on physics. My question is: is it ok for a person like me, frequently getting into trouble when studying some advanced topics on Quantum Mechanics or General Relativity? In my course, we are learning about basic...
  22. secur

    I No Special Reason for Thinking Consciousness Special in QM

    There's no question that MWI is an important part of QM, whether we agree with it or not. And consciousness is essential for MWI, it can't be defined without it. For instance, Stanford Encyclopedia https://plato.stanford.edu/entries/qm-manyworlds/#2.2 says: ***********************************...
  23. J

    I Hermitian Operators in QM

    I have been following a series of on-line lectures by Dr Physics A. He clearly describes what Hermitian operators for polarization and spin are and what they do. But when he gets to the position and momentum operators I am rather lost. They are no longer represented by square matrices. The...
  24. GiuseppeP

    I Entanglement in QM - Understanding Instant Correlation & Randomness

    Hello, I have red on the web about the entanglement, but there is one thing that it is not clear to me: to explain it, why we cannot just say that randomness and correlation between the two entangled particles is happening at instant of their creation and from that time on they stay in that...
  25. U

    Need recommendation for beginner QM book

    Can someone give me a link to a site that can teach me from the start in finding it hard to find something as you would in say a class room
  26. P

    Quantum QM Textbooks (Education + Research purposes)

    Hello fellow physics enthusiasts! Apologies in advance, I wanted to make the most out of this post and so it ended up being quite long + detailed. I thank you for your patience and any efforts to help! What I'm looking for: TWO good Quantum Mechanics textbooks for the rest of my undergraduate...
  27. Vitani11

    Units of orbital angular momentum QM?

    Homework Statement A hydrogen atom is in the 3d state. Determine the orbital angular momentum. Homework Equations unnecessary The Attempt at a Solution I already know how to figure this out and have an answer but WHAT ARE the units?! It's not kgm/s^2!
  28. Grinkle

    B Does QM allow for entropy decrease of a closed system?

    I recently posted a statement that according to the 2nd law of thermodynamics, entropy always increases in a closed system, no matter how small the time interval is that one looks at. I think that is true for classical thermodynamic models, please correct me if I am wrong. Does quantum...
  29. B

    First Order Perturbation Theory - QM

    Homework Statement The ground state energy of the 1D harmonic oscillator with angular frequency ##\omega## is ##E_0 = \frac{\hbar \omega}{2}##. The angular frequency is perturbed by a small amount ##\delta \omega##. Use first order perturbation theory to estimate the ground state energy of the...
  30. Tspirit

    I Is dψ/dx zero when x is infinite in QM?

    In QM, we all know that the wavefunction ψ is zero when x is infinite. However, Is dψ/dx also zero when x is infinite? And the d2ψ/dx2?
  31. K

    Quantum Self-Learning QM: Is It Difficult?

    (Sorry for my bad English.) I've been reading severous QM textbooks that people say are good to learn QM: Sakurai, Shankar, Thankappan. I start reading and understand things in principle, but then I don't get anymore. The only book I think really helped me was QM by McIntyre but it goes not deep...
  32. W

    I QM has never made a prediction contradicted by experiment

    I have had many people claim that QM has never made a prediction that has been contradicted by experiment. Yet as i understand it Qm predicts the vacuum energy density is 10^122 erg per cubic cm whereas the measured energy density of 10^-8 erg per cubic cm. So how is that not an example of an...
  33. J

    A What physics would look like if QM had not been invented?

    It is hard to imagine that classical mechanics could develop in an essentially different way - Newton, derivatives, Coulomb etc. This seems a very natural evolution of understanding and intuitions. However, the early XX century physics: axioms of QM (and GRT) seems strongly dependent on a...
  34. K

    B QM: Self-Studying vs. University Classes

    (Sorry for my poor English.) How much of theory should one know in order to understand Gross-Pitaevskii equation, Fermi curves, metric tensor and so on? Is it necessary to took classes at the university or is it possible to learn those subjects just by self studying?
  35. K

    I Understanding Complex Coefficients in Quantum Mechanics

    Ok. I will try to do my best to explain you what is my doubt. I'm not a native English Speaker and the book I was reading is not in English Lang, but I've translated it to English. In a spin-1/2 Stern Gerlach-like experiment, we can express the ket representing the spin component of the...
  36. entropy1

    I Discovery shakes up QM (or not....)

    A yet unverified experiment may mean a paradigm shift in QM: http://advances.sciencemag.org/content/2/2/e1501466.full http://futurism.com/new-evidence-could-break-the-standard-view-of-quantum-mechanics/ What do you make of this?
  37. binbagsss

    QM Bra & Ket Linear Algebra Hermitian operator proof -- quick question

    Homework Statement Hi, Just watching Susskind's quantum mechanics lecture notes, I have a couple of questions from his third lecture: Homework Equations [/B] 1) At 25:20 he says that ## <A|\hat{H}|A>=<A|\hat{H}|A>^*## [1] ##<=>## ##<B|\hat{H}|A>=<A|\hat{H}|B>^*=## [2] where ##A## and ##B##...
  38. gasar8

    Two particles in magnetic field QM

    Homework Statement Two particles with spin S_1={1 \over 2} and S_2={1 \over 2} are at t=0 in a state with S=0. a) Find wave function at t=0 in S_{1z},, S_{2z} basis. b) Second particle is in a magnetic field B = (\sin\theta,0,\cos\theta), the Hamiltonian is H=\lambda \vec{S_2} \cdot \vec{B}...
  39. K

    Programs Graduate Program in QM: Researching Foundations of Quantum Mechanics

    What are strong graduate programs for researching foundations of quantum mechanics?
  40. D

    Quantum Looking for QM book on symmetry and groups

    Hi. I am looking for a QM book that covers symmetry , time-reversal , angular momentum representations in SO(3). I have a few books and most of them don't have much detail on these subjects.The main one that does is Sakurai. Any other suggestions ? Thanks
  41. gulfcoastfella

    I What's the motivation for bracket notation in QM?

    I took a semester of QM as an undergrad engineering major, and I don't recall the motivation for replacing traditional vector notation with bracket notation. Can someone enlighten me? Thank you.
  42. F

    Looking for Quantum Mechanics Courses? Any Recommendations?

    Hello guys, I just finish my first year at uni Phys and Maths,and would like to self teach Quantum Mechanics during this summer, as so I was wondering if you guys could suggest me any good full online courses/ lectures available on the web . I have found from neptel from Oxford Bookwise...
  43. K

    Quantum Struggling with QM lectures? Need a helpful self-teaching book?

    I'm looking for help teaching myself QM. I am taking a a third year course in my degree in QM but I can't follow the lecturers notes/teaching style. The topics we will be covering are; Introduction: experimental facts that led to the development of quantum theory Waves and particles...
  44. N

    I Clarification of the postulates of QM

    So I'm taking a course in QM right now and would like some clarifications on the postulates of QM, mainly I'm looking for physical intuition and may be for someone to clear some misconceptions if I have any, so without further ado let's begin: first I'd like to state the postulates as I'm...
  45. H

    Mean value of energy <E> for a QM state?

    Homework Statement If the system is in a state |ψ> = 1/sqrt(6) |v1> + 1/sqrt(3) |v2> - i/sqrt(2) |v3> with Hamiltonian satisfying H|vj> = (2-j)a|vj> Find the mean value of energy <E> and the root mean square deviation √(<E2> - <E>2 ) that would result from making a number of measurements of...
  46. J

    I QM - Ladder Operator QHO - factorization

    Hi, quick question with A being the lowering operator and A† the raising operator for a QHO (A A† - 1 + 1/2) ħω [Aψ] = A (A† A - 1 + 1/2) ħω ψ By taking out a factor of A. Why has the ordering of A A† swapped around? I would have thought taking out a factor of A would leave it as A (A† - 1 +...
  47. entropy1

    B After the 'Theoretical minimum' series, what is essential to know about QM?

    The adagium of most quantumphysics-afficionado's is: "Shut up and calculate" - 'learn the formalism'. So I started with Leonard Susskind's 'Theoretical minimum' textbooks. So now I know a little (very little) about the formalism, I started to wonder to which extent I have to go to educate...
  48. Kara386

    I What Causes a Measurement to Change the State of a System in Quantum Mechanics?

    I read that measurement changes things because we have to bounce photons off an object to 'see' it and that changes its position, momentum etc... But on the other hand, Griffiths' QM book seems to suggest we don't know what it is about measurement that changes the state of something. We don't...
  49. Einstein's Cat

    B Is QM an Intrinsic Physical Theory?

    The purpose of the tread is to provoke varied opinions and to allow those who obtain such views to offer supporting evidence. The question is this: "to what extent is QM an intrinsic physical theory?"
  50. J

    Looking for Mathematically Rigorous QM Textbooks? Any Suggestions?

    Upon searching in this forum, i have found discussions about the standard undergraduate textbooks on QM not being so good in teaching you the foundations properly. A good example is the difference between Hermitian and self-adjoint operators. Some people are saying that we should study QM from a...
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