Dear Forum,<br><br>I have some questions on setting the parameter set (ecfixed,q2sigma,qcurtz) in cp.x of espresso-4.2.1.<br>I use SiO2 of example18 in source code.<br><br>I found that when I adjust (ecfixed,q2sigma,qcurtz) the average volume at equilibrium state varies.<br>
How do think of this volume disagreement? <br>
What is the criteria to select the best (ecfixed,q2sigma,qcurtz)  value?<br><br><br><br>I summarized my results in detail following.  <br><br><br>In this code, the original value is  (ecfixed=16,q2sigma=2,qcutz=150).<br>
I did adjustment with all other parameter fixed as the same to check the pressure, volume, and total energy at equilibrium state of 10Kbar-10K. <br>I adjust ecfixed first, then q2sigma, then qcutz.<br><br>1. adjusting ecfixed.<br>
ecfixed (with q2sigma=2,qcutz=150)        average  volume              volume vibarion<br>17.1                                                                1340                     small <br>17.0                                                                1350                     small                                                                 <br>
16.9                                                                1360                     small <br>16.8                                                                1370                     small <br>16                                                                   1330                     larger  <br>
19                                                                   1390                     larger  <br>All above have similar pressure vibration.<br>The larger ecfixed, the lower total energy.<br><br>How do think of this volume discrepancy? Is this satisfying with inaccuracy?<br>
What is the criteria to select the best ecfixed value?<br>Now, without anyother base, personally, I think that ecfixed=16.8 has small volume vibration and it has the largest volume among those with small volume vibration.<br>
Maybe ecfixed=16.8 it is. <br><br>2. adjust q2sigma<br>q2sigma (with ecfixed=16.8,qcutz=150)           volume                 volume vibration<br>
1.6                                                                1420                           larger<br>
1.8                                                                1345
                           larger                                      <br>
2.0                                                                1370                           small<br>2.2                                                                1380                           larger<br>
2.4                                                                1370                           larger<br>
2.6                                                                1330                           larger<br>All above have similar pressure vibration.<br>
The smaller q2sigma, the lower total energy.<br><br>I think that q2sigma=2 is the best because it has smallest volume vibration. <br>Right?<br><br>3. adjusting qcutz<br>
qcutz (with ecfixed=16.8,q2sigma=2)           volume                 volume vibration<br>

50                                                                1310                           small<br>

80                                                                1335
                           small                                      <br>

120                                                              1360                           small<br>
150                                                              1370                           small<br>

180                                                              1380                           small<br>

400                                                              1370                           small<br>
All above have similar pressure vibration.<br>

The smaller qcutz, the lower total energy.<br><br>They have similar results except the volume discrepancy. <br>So what is the criteria to select ? <br><br><br><br>In sum, regarding the volume discrepancy, how to select the best value for  (ecfixed,q2sigma,qcurtz) ?<br>
<br><br>Thanks for you attention.<br>Any comment is grateful.<br><br><br>WANG Riping <br>2010.1.7 <br><br> <br>-- <br><div>******************************************************************************<br>WANG Riping<br>Ph.D student, </div>

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