<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Dear GMX developers,<div class=""><br class=""></div><div class="">First of all thanks for your continuos efforts in making gromacs better.</div><div class=""><br class=""></div><div class="">I have done a very basic calculation with the new gromacs-2016-beta1 and I think there is a bug somewhere</div><div class="">in the force calculation. (all calculations are run using 1 processor, no mpi, no openmp, no gpus)</div><div class=""><br class=""></div><div class="">I have run a calculation on alanine dipeptide:</div><div class=""><br class=""></div><div class="">1) GMX507: 2 steps using 1 processor (avx2 simd and nothing else, the tpr is generated with gromacs 507)</div><div class="">2) rerun with GMX512 on the trr file</div><div class="">3) rerun with GMX2016 on the trr file</div><div class="">and then I compared forces, velocities and energies, and while they are essentially the same for the step 0</div><div class="">in the case of GMX507 and 512 they are not the same anymore for GMX2016b1.</div><div class=""><br class=""></div><div class="">The differences are not huge, even if from a rerun I would expect no differences at all, in particular on the</div><div class="">forces, so I have run a metadynamics calculation from the same tpr file and using the three codes</div><div class="">patched with plumed to see the effect on the free energy along the phi torsion angle, and unfortunately the </div><div class="">difference is quite significant.</div><div class=""><br class=""></div><div class="">to be sure I repeated the same tests on a linux workstation (the first test was on my macbook), so different</div><div class="">compilers and so on, but always avx2 simd)</div><div class=""><br class=""></div><div class="">here you can find everything (all the files I have generated):</div><div class=""><a href="https://dl.dropboxusercontent.com/u/1349402/bug-report-aladip-2016.tgz" class="">https://dl.dropboxusercontent.com/u/1349402/bug-report-aladip-2016.tgz</a></div><div class=""><br class=""></div><div class="">I hope this helps.</div><div class=""><br class=""></div><div class="">Best,</div><div class="">Carlo</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div></body></html>