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+====== Hartree-Fock exchange for the dihydrogen cation ======
+The goal of this exercise is to calculate the dissociate enery profile of the [[wp>Dihydrogen_cation|dihydrogen cation]] $H_2^+$.
+The curve should be record for three different fractions of Hartree-Fock exchange:
+  * 0% HF exchange, 100% PBE exchange
+  * 50% HF exchange, 50% PBE exchange
+  * 100% HF exchange, 0% PBE exchange
+===== Questions =====
+  * Report the dissociation curve for the $H^+_2$ ion, for the three fractions of Hartree-Fock exchange employed.
+===== Required Files =====
+==== Input File ====
+<code - H2ion.inp>
+&GLOBAL
+  PROJECT H2ion
+  RUN_TYPE ENERGY
+&END GLOBAL
+&FORCE_EVAL
+  METHOD Quickstep
+  &DFT
+    ! data-files for basis-set and pseudo-potentials
+    BASIS_SET_FILE_NAME HFX_BASIS
+    POTENTIAL_FILE_NAME POTENTIAL
+    &MGRID              ! resolution of finest grid, higher values give more accurate results
+      CUTOFF 400
+    &END MGRID
+    &POISSON            ! Non periodic calculation needs Poisson solver
+      PERIODIC NONE
+      PSOLVER  WAVELET
+    &END POISSON
+    &SCF                 ! need to achieve SCF-convergence at large H-H distances
+      &OT
+        PRECONDITIONER FULL_ALL
+      &END OT
+    &END SCF
+    CHARGE 1             ! H2 cation is positively charged...
+    MULTIPLICITY 2       ! ...and therefore in a doublet spin state...
+    LSD                  ! ...so that we have to run an unrestricted Kohn-Sham calculation
+    &XC                  ! Exchange and Correlation settings
+      &XC_FUNCTIONAL
+        &PBE
+          SCALE_X 0.5   ! amount of PBE eXchange (vary consistently with the HF/FRACTION)
+          SCALE_C 1.0    ! amount of PBE Correlation. DO NOT VARY!!!!!!
+        &END
+      &END XC_FUNCTIONAL
+      &HF
+        FRACTION 0.5    ! amount of HF eXchange (vary consistently with PBE/SCALE_X)
+        &SCREENING
+          EPS_SCHWARZ 1.0E-10
+        &END
+      &END HF
+    &END XC
+  &END DFT
+  &SUBSYS
+    &TOPOLOGY
+      &CENTER_COORDINATES  ! keep atoms away from box borders, would confuse Poisson solver
+      &END
+    &END
+    &CELL
+      ABC 15.0 15.0 15.0
+      PERIODIC NONE  ! Non periodic calculation.
+    &END CELL
+    &COORD
+       H    0.0   0.0   0.0
+       H    0.42  0.0   0.0  ! vary H-H distance to record dissociation profile
+    &END COORD
+    &KIND H
+      BASIS_SET DZVP-GTH
+      POTENTIAL GTH-PBE-q1
+    &END KIND
+  &END SUBSYS
+&END FORCE_EVAL
+</code>
+==== Basis Set ====
+<code - HFX_BASIS>
+H DZVP-GTH
+  0  0  3  1
+.3744350009  -0.0283380461
+.8058681460  -0.1333810052
+.4852528328  -0.3995676063
+  0  0  1  1
+.1658236932   1.0000000000
+  1  1  1  1
+.7270000000   1.0000000000
+#
+C DZVP-GTH
+  0  1  3  1  1
+.3362376436   0.1490797872  -0.0878123619
+.2881838513  -0.0292640031  -0.2775560300
+.4037767149  -0.6882040510  -0.4712295093
+  0  1  1  1  1
+.1187877657   1.0000000000   1.0000000000
+  2  2  1  1
+.5500000000   1.0000000000
+</code>
+==== Pseudo Potential ====
+<code - POTENTIAL>
+H GTH-PBE-q1
+.20000000    2    -4.17890044     0.72446331
+#
+C GTH-PBE-q4
+    2
+.33847124    2    -8.80367398     1.33921085
+.30257575    1     9.62248665
+.29150694    0
+</code>