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 Sep 30 awarded Explainer Sep 8 awarded Enthusiast Jul 31 comment What colour is nothing? That kind of black with bright speckles you see (or at least I do) in the darkest room of the house in the middle of the night. But is this what your eyes see or what the brain sees? Jul 29 awarded Scholar Jul 29 accepted Multiplicity of eigenvalues of angular momentum Jul 29 comment Multiplicity of eigenvalues of angular momentum I guess you are right. What fooled be is that in other cases, when $-\hslash$ is added the number times changes by 1, and I assumed that would always be the case. Of course, when $k_1 \gg k_1$ there will be quite a few eigenvalues in the middle of the sequence with $k_2+1$ multiplicity... Jul 28 asked Multiplicity of eigenvalues of angular momentum Jul 23 comment What is the physical meaning of diffusion coefficient? Do you mean that the dimenisionality of $D$ is $\text{length}^2/\text{time}$? That's not a velocity, it's in any case a surface velocity (rate of increase of area). Probably the most useful interpretation is just the straightforward one: it's the constant of proportionality between the gradient of concentration and the flux of substance. Look here for a dimensional analysis. Jul 20 answered Why Gravity attracts all objects with the same speed? Jul 19 comment Optical Drive Physics Have you checked the Wikipedia? Jul 19 answered What physical forces give rise to the peculiar bond angle of hydrogen peroxide? Jul 17 comment Where do the terms microcanonical, canonical and grand canonical (ensemble) come from? Maybe in his "Equilibrium" paper? Jul 17 comment Where do the terms microcanonical, canonical and grand canonical (ensemble) come from? I was trying to read something in that book, but through Google in a reprint... so the right pages were not available. This is what it says in p. xi: "We consider especially ensembles of systems in which the index (or logarithm) of probability of phase is a linear function of the energy. This distribution, on account of its unique importance in the theory of statistical equilibrium, I have ventured to call canonical, and the divisor of the energy, the modulus of distribution." Jul 17 comment Where do the terms microcanonical, canonical and grand canonical (ensemble) come from? I think it was J. W. Gibbs who first defined them, and probably coined the terms... Jul 17 comment Adding rotation to internal coordinates I'm not trying to find a global minimum, and there's a large body of literature and experience supporting internal coordinates for chemical systems. The energy of molecules can be most easily rationalized in terms of bond distances and angles, so it makes every sense to use these coordinates to describe their geometry and therefore to optimize it. Read the paper I cited in the question, if you have access to it, and you'll see it works. Jul 16 comment Adding rotation to internal coordinates I'm doing optimization with either (quasi-)Newton-Raphson, RFO, conjugated gradients... These are algorithms that use the derivative of the energy with respect to the coordinates, and work by iteratively estimating the location of a (local) minimum. Maybe that's where the misunderstanding comes from. I'm not sampling the space of all possible configurations trying to find a global minimum, I'm rather following the potential energy surface to find a sensible minimum (or saddle point, with the appropriate method). Jul 16 comment Adding rotation to internal coordinates I'm not working with a single specific system or kind of system. I'm programming a general geometry optimizer for potentially any system (although I don't think I'll use it for more than ~100 atoms). Something like most quantum chemistry programs have (why I don't use the available programs is another story). Jul 16 comment Adding rotation to internal coordinates I'm sorry that's what you think. But as I said, in the comments to my question, Cartesian coordinates, while possible, are generally not the most efficient. It is true this is not a life-or-death question, I can have the job done by using Cartesian coordinates, but since I already have a program that works very nicely with internal coordinates when there is translational and rotational invariance, I just needed to add these degrees of freedom. Jul 16 awarded Commentator Jul 16 comment Why do we still not have an exact definition for a kilogram? @JerrySchirmer, except that you'd need to define the mole first, and it is currently defined in relation to the kilogram. An approach that I believe is under discussion is fixing Avogadro's constant to some exact number, and then yes, that would be an appropriate definition... but still difficult to reproduce in a lab.