<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Dear Mayank, <div class=""><br class=""></div><div class="">the y-axis of your conserved quantity plot is too coarse to judge if energy conservation is an issue at all. </div><div class="">Possibly it is already good enough, but in any case the integration time step error is not <a href="http://s.th" class="">s.th</a>. you should </div><div class="">be concerned first and foremost, hence you can safely increase TIMESTEP. Also, since energy is an extensive </div><div class="">property, in my opinion the best way to look at the conserved quantity is in units of Kelvin per atom and ps. </div><div class="">The potential energy is a matter of your initial configuration and only shows that you haven’t equilibrated </div><div class="">as yet … </div><div class=""><br class=""></div><div class="">Cheers, </div><div class="">Thomas </div><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">Am 04.08.2020 um 08:52 schrieb mayank...@<a href="http://gmail.com" class="">gmail.com</a> <<a href="mailto:mayan...@gmail.com" class="">mayan...@gmail.com</a>>:</div><br class="Apple-interchange-newline"><div class="">Hi,<div class=""><br class=""></div><div class="">I am trying a NVT MD simulation for a QM/MM system with QM water molecules surrounding a spatially fixed MM molecule, which are in turn surrounded by MM water molecules. I am using a time step of 5 a.u. (for particular reasons) and the QM box is much smaller than compared to full MM box so within the timescale of MD simulation the QM molecules do not cross the QM box boundary. I am attaching the relevant sections of input file:</div><div class="">----</div><div class=""><div class=""> &MOTION</div><div class=""> &MD</div><div class=""> ENSEMBLE NVT</div><div class=""> STEPS 10000</div><div class=""> TIMESTEP 1.21E-01</div><div class=""> TEMPERATURE 3.00E+02<br class=""></div><div class=""> &THERMOSTAT</div><div class=""> TYPE NOSE</div><div class=""> REGION DEFINED</div><div class=""> &DEFINE_REGION</div><div class=""> QM_SUBSYS ATOMIC</div><div class=""> &END DEFINE_REGION</div><div class=""> &DEFINE_REGION</div><div class=""> MM_SUBSYS ATOMIC</div><div class=""> &END DEFINE_REGION</div><div class=""> &NOSE</div><div class=""> TIMECON 9.99E+02</div><div class=""> &END NOSE</div><div class=""> &END THERMOSTAT</div></div><div class=""><br class=""></div><div class=""><div class=""> &FORCE_EVAL</div><div class=""> METHOD QMMM</div><div class=""> &DFT</div><div class=""> BASIS_SET_FILE_NAME ./GTH_BASIS_SETS</div><div class=""> POTENTIAL_FILE_NAME ./GTH_POTENTIALS</div><div class=""> CHARGE 0</div><div class=""> &SCF</div><div class=""> MAX_SCF 60</div><div class=""> EPS_SCF 2.0E-07</div><div class=""> SCF_GUESS RESTART</div><div class=""> &OT T</div><div class=""> MINIMIZER DIIS</div><div class=""> PRECONDITIONER FULL_SINGLE_INVERSE</div><div class=""> &END OT</div><div class=""> &OUTER_SCF T</div><div class=""> EPS_SCF 2.0E-07</div><div class=""> MAX_SCF 40</div><div class=""> &END OUTER_SCF</div><div class=""> &PRINT</div><div class=""> &RESTART SILENT</div><div class=""> BACKUP_COPIES 1</div><div class=""> &END RESTART</div><div class=""> &END PRINT</div><div class=""> &END SCF</div><div class=""> &QS</div><div class=""> EXTRAPOLATION ASPC</div><div class=""> EXTRAPOLATION_ORDER 4</div><div class=""> &END QS</div><div class=""> &MGRID</div><div class=""> CUTOFF 3.2E+02</div><div class=""> COMMENSURATE T</div><div class=""> &END MGRID</div><div class=""> &XC</div><div class=""> DENSITY_CUTOFF 1.0E-10</div><div class=""> GRADIENT_CUTOFF 1.0E-10</div><div class=""> TAU_CUTOFF 1.0E-10</div><div class=""> &XC_GRID</div><div class=""> XC_SMOOTH_RHO NN50</div><div class=""> XC_DERIV SPLINE2_SMOOTH</div><div class=""> &END XC_GRID</div><div class=""> &XC_FUNCTIONAL NO_SHORTCUT</div><div class=""> &PBE T</div><div class=""> PARAMETRIZATION REVPBE</div><div class=""> &END PBE</div><div class=""> &END XC_FUNCTIONAL</div><div class=""> &VDW_POTENTIAL</div><div class=""> POTENTIAL_TYPE PAIR_POTENTIAL</div><div class=""> &PAIR_POTENTIAL</div><div class=""> R_CUTOFF 1.9E+01</div><div class=""> TYPE DFTD3</div><div class=""> PARAMETER_FILE_NAME ./dftd3.dat</div><div class=""> REFERENCE_FUNCTIONAL revPBE</div><div class=""> CALCULATE_C9_TERM T</div><div class=""> REFERENCE_C9_TERM T</div><div class=""> LONG_RANGE_CORRECTION F</div><div class=""> &END PAIR_POTENTIAL</div><div class=""> &END VDW_POTENTIAL</div><div class=""> &END XC</div><div class=""> &END DFT</div><div class=""> &MM</div><div class=""> &FORCEFIELD</div><div class=""> ...</div><div class=""> &END FORCEFIELD</div><div class=""> &POISSON</div><div class=""> &EWALD</div><div class=""> EWALD_TYPE SPME</div><div class=""> RCUT 1.2E+01</div><div class=""> ALPHA 2.917E-01</div><div class=""> GMAX 75</div><div class=""> O_SPLINE 4</div><div class=""> &END EWALD</div><div class=""> &END POISSON</div><div class=""> &END MM</div><div class=""> &QMMM</div><div class=""> E_COUPL GAUSS</div><div class=""> USE_GEEP_LIB 6</div><div class=""> NOCOMPATIBILITY T</div><div class=""> CENTER NEVER</div><div class=""> INITIAL_TRANSLATION_VECTOR 0.0E+00 0.0E+00 0.0E+00</div><div class=""> &QM_KIND O</div><div class=""> MM_INDEX ...</div><div class=""> &END QM_KIND</div><div class=""> &QM_KIND H</div><div class=""> MM_INDEX ...</div><div class=""> &END QM_KIND</div><div class=""> &MM_KIND H</div><div class=""> RADIUS 4.4E-01</div><div class=""> &END MM_KIND</div><div class=""> &MM_KIND O</div><div class=""> RADIUS 1.2E+00</div><div class=""> &END MM_KIND</div><div class=""> .....</div><div class=""> &CELL</div><div class=""> ABC 4.0E+01 3.0E+01 3.0E+01</div><div class=""> PERIODIC XYZ</div><div class=""> &END CELL</div><div class=""> &PERIODIC</div><div class=""> GMAX 5.0E-01</div><div class=""> &MULTIPOLE ON</div><div class=""> RCUT 1.2E+01</div><div class=""> EWALD_PRECISION 1.0E-08</div><div class=""> ANALYTICAL_GTERM T</div><div class=""> &END MULTIPOLE</div><div class=""> &END PERIODIC</div><div class=""> &END QMMM</div><div class=""> &SUBSYS</div><div class=""> &CELL</div><div class=""> A 5.0E+01 0.0E+00 0.0E+00</div><div class=""> B 0.0E+00 4.0E+01 0.0E+00 </div><div class=""> C 0.0E+00 0.0E+00 4.0E+01</div><div class=""> MULTIPLE_UNIT_CELL 1 1 1</div><div class=""> &END CELL</div><div class=""> &COORD</div></div><div class=""><br class=""></div><div class="">-------</div><div class=""><br class=""></div><div class="">With this setup for the production run I obtained the following trends for the potential and constant quantity as the graphs attached here. I have tested the QM setup for a fully QM calculation on bulk water, which works fine. My main aim here is to reduce the energy drift for the QM/MM system, so is there any other aspect of the calculation should I take care to reduce the drift? Any suggested corrections for the QM/MM file would also be helpful.</div><div class=""><br class=""></div><div class="">Best Regards,</div><div class="">Mayank Dodia</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class="webkit-block-placeholder"></div> -- <br class=""> You received this message because you are subscribed to the Google Groups "cp2k" group.<br class=""> To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:cp...@googlegroups.com" class="">cp...@googlegroups.com</a>.<br class=""> To view this discussion on the web visit <a href="https://groups.google.com/d/msgid/cp2k/63b0f162-c684-481d-884a-f10890c1aa91n%40googlegroups.com?utm_medium=email&utm_source=footer" class="">https://groups.google.com/d/msgid/cp2k/63b0f162-c684-481d-884a-f10890c1aa91n%40googlegroups.com</a>.<br class=""> <span id="cid:2807a701-c1a0-4d7d-b4d3-1350156bc5a7"><const_quantity.png></span><span id="cid:43430373-5279-41af-9507-cb7c773c04e9"><pot_enr.png></span></div></blockquote></div><br class=""></div><br class=""><br class=""><div class=""> <div style="color: rgb(0, 0, 0); letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-east-asian: normal; font-variant-position: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; orphans: 2; widows: 2;" class="">==============================</div><div style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-east-asian: normal; font-variant-position: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; orphans: 2; widows: 2;" class="">Thomas D. 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