So, codes like Gaussian, Jaguar, ADF, NWChem, ORCA, etc. The accuracy of any quadrature approach depends on the density of the grid points, and, of course, the cost of the integral evaluation also goes up with the number of grid points. The so-called analytic derivatives and second derivatives are thus NOT analytic derivatives of the correct integrals, they are analytic derivatives of the quadrature schemes for these integrals. Instead, the integrals are solved via a quadrature procedure over a 3-dimensional grid. Most (if not all) modern DFT functionals do NOT permit an analytic evaluation of the necessary volume integrals of the exchange-correlation potential. Such procedures can indeed be effective, but only if the imaginary frequency is really there… Left unaddressed (at least in the most recent iteration of this thread - I have vague memories that others may have posted to CCL on this point before) is the possibility that the unwanted imaginary frequency is an artifact of the quadrature grid used in a DFT calculation (recent posters have not actually specified their level of electronic structure theory, but given the prevalence of DFT in modern calculations, one suspects this was indeed their choice). On a more substantial front, several posters have offered suggestions for removing the “minor” imaginary frequency by perturbing geometries along the predicted normal mode, reoptimizing, generally jiggling structures, etc. It is a historical curiosity, probably relating to the difference between FORTRAN floating point and character variables, that most codes print the specific frequencies as negative numbers rather than appending Euler’s “i” after the magnitude. In the quantum mechanical harmonic oscillator (QMHO) approximation, the frequency is related to the square root of the force constant, and hence it is imaginary. It is the computed force constant that is negative. I browsed one of the mailing list, and I got a nice explanation about the scientific term of negative frequency written by Christopher Crammer:įirst, at the risk of sounding needlessly didactic, the frequencies are not “negative,” they are imaginary. Using Molden software, you can nicely see what happen to the molecule in the negative frequency. So, it could be TS of rotation, conformation, addition, insersion, and so on. In Gaussian calculation, it is known by the existence of single negative value of frequency. If you really need this approach, then you need to do it manually by extracting the geometries (and possibly the MO) and perform the frequency job on top of it.In reaction kinetic calculation, the transition structure is necessary to calculate the kinetic rate constant. You are employing the harmonic oscillator approximation, which will break down if you are not using a local minimum (or at least a stationary point). Additionally frequency calculations are only meaningful at well converged local minima structures. As TAR86 pointed out, Raman intensities are expensive. Only run compound jobs if you are 100% sure they cannot fail (and even then it's questionable).įrom a theoretical standpoint, you should consider how much sense your approach makes. Every well organised queuing system should be able to run jobs one after the other when specified (i.e. Lastly, with the introduction of %oldchk= in in revision D.01, it is completely superfluous. It's the dream of every administrator (and co-worker) when you block resources you don't need for an extended period of time just to avoid a little more waiting. In such cases, you also give up control over the validity of your checkpoint-file, which you might want to reuse.Īnother issue is that these are basically two jobs running one after another and in most cases need different memory requirements, are more effective with another setup. Additionally, you do not really have any control about what happens if something happens to fail.
If you are using any kind of IOp, then the probability that the second part of your job is doing what you want is low. Using compound jobs is unfortunately very common among computational chemists, but they should always be viewed critically. No, there is no automation for this, and there should not be.
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