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+====== Reaction Energy ======
+In this exercise, you will calculate the reaction energy for the **methane combustion** reaction:
+\[ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \]
+Reaction energy:
+\[ \sum E_\text{products} -  \sum E_\text{rectants} = \left (2\cdot E_{H_2O} + E_{CO_2} \right) - \left(E_{CH_4} + 2\cdot E_{O_2}\right) \]
+Ground state oxygen, O$_2$, is a triplet diradical, a property which can explain why liquid oxygen is paramagnetic and attracted to the poles of a magnet.
+{{ o3.png?600 |
+}}
+For this reason, to get the energy of the O$_2$ molecule, a LSD calculation is required.
+===== 1.Step =====
+Run a single point calculation for CH$_4$, using the given input file.
+Note that the file contains explicit basis sets and potential for all-electron calculations. An explanation of the basis set formats is given here: [[basis_sets|Basis Sets]]
+<code - CH4.inp >
+&GLOBAL
+  PROJECT CH4
+  RUN_TYPE ENERGY
+  PRINT_LEVEL MEDIUM
+&END GLOBAL
+&FORCE_EVAL
+  METHOD Quickstep              ! Electronic structure method (DFT,...)
+  &DFT
+    &POISSON                    ! Solver requested for non periodic calculations
+      PERIODIC NONE
+      PSOLVER  WAVELET          ! Type of solver
+    &END POISSON
+    &QS                         ! Parameters needed to set up the Quickstep framework
+      METHOD GAPW               ! Method: gaussian and augmented plane waves
+    &END QS
+    &XC                        ! Parametes needed to compute the electronic exchange potential
+      &XC_FUNCTIONAL NONE      ! No xc functional
+      &END XC_FUNCTIONAL
+      &HF                      ! Hartree Fock exchange. In this case is 100% (no fraction specified).
+        &SCREENING             ! Screening of the electronic repulsion up to the given threshold.
+          EPS_SCHWARZ 1.0E-10  ! Threshold specification
+        &END SCREENING
+      &END HF
+    &END XC
+  &END DFT
+  &SUBSYS
+    &CELL
+      ABC 10 10 10
+      PERIODIC NONE              ! Non periodic calculations. That's why the POISSON section is needed
+    &END CELL
+    &TOPOLOGY                    ! Section used to center the atomic coordinates in the given box. Useful for big molecules
+      &CENTER_COORDINATES
+      &END
+    &END
+    &COORD
+  C         4.6425962273        5.0574874650        5.2069537560
+  H         5.7240587065        5.0555482951        5.2189766147
+  H         4.2766068912        5.8773176685        5.8100567767
+  H         4.2759350196        4.1226994019        5.6087492584
+  H         4.2938562590        5.1744089096        4.1899119266
+    &END COORD
+    &KIND H                                      ! potential and basis for H
+      &BASIS
+  0  0  3  1
+.25200000          0.02282200
+.86870000          0.15564000
+.41821000          0.48898000
+  0  0  1  1
+.10610000          1.00000000
+  1  1  1  1
+.00000000          1.00000000
+      &END
+      POTENTIAL ALL
+      &POTENTIAL
+    0    0
+.20000000    0
+      &END
+    &END KIND
+    &KIND C                                      ! potential and basis for C
+      &BASIS
+  0  0  6  2
+.60000000          0.00557360          0.00000000
+.57000000          0.04149600         -0.00027440
+.83900000          0.18263000         -0.00255830
+.81800000          0.46129000         -0.03337500
+.55670000          0.44931000         -0.08730500
+.54258000          0.00000000          0.53415000
+  0  0  1  1
+.16058000          1.00000000
+  1  1  3  1
+.14260000          0.04449900
+.92980000          0.23108000
+.52522000          0.51227000
+  1  1  1  1
+.13608000          1.00000000
+  2  2  1  1
+.80000000          1.00000000
+      &END
+      POTENTIAL ALL
+      &POTENTIAL
+    2    0
+.34883045    0
+      &END
+    &END KIND
+  &END SUBSYS
+&END FORCE_EVAL
+</code>
+If the calculation was performed correctly, the total energy of the CH$_4$ molecule is printed in the output file.
+<code>
+  **** **** ******  **  PROGRAM STARTED AT
+ ***** ** ***  *** **   PROGRAM STARTED ON
+ **    ****   ******    PROGRAM STARTED BY
+ ***** **    ** ** **   PROGRAM PROCESS ID
+  **** **  *******  **  PROGRAM STARTED IN
+.....
+ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
+.....
+  **** **** ******  **  PROGRAM ENDED AT
+ ***** ** ***  *** **   PROGRAM RAN ON
+ **    ****   ******    PROGRAM RAN BY
+ ***** **    ** ** **   PROGRAM PROCESS ID
+  **** **  *******  **  PROGRAM STOPPED IN
+</code>
+===== 2.Step =====
+Modify the input in order to perform the same calculation for:
+  * H$_2$O
+  * CO$_2$
+  * O$_2$ triplet
+<note tip> Atomic coordinates for all the molecules, POTENTIAL and BASIS SET for KIND O are given at the end of the exercise. </note>
+<note important>Remember that the O2 ground state is a triplet state, with non paired electrons.
+ MULTIPLICITY=2S+1=3. </note>
+For O2 triplet, the LSD and MULTIPLICITY keywords are needed in the DFT section:
+<code>
+  METHOD Quickstep
+   &DFT
+     LSD                           ! Requests a spin-polarized calculation for unpaired electrons
+     MULTIPLICITY 3                ! Multiplicity = 2S+1 (S= total spin momentum)
+  ...
+</code>
+Another example can be found here [[basis_sets|Basis Sets]]
+===== 3.Step =====
+At the end, you should get a table like:
+^ Species ^ Total Energy ^
+| CH$_4$  | ...          |
+| O$_2$   | ...          |
+| H$_2$O  | ...          |
+| CO$_2$  | ...          |
+Now you can compute the overall reaction energy.
+===== Questions =====
+  - What are the total energies of O$_2$, H$_2$O, CO$_2$, and CH$_4$?
+  - What is the overall reaction energy of the CH$_4$ combustion?
+  - **(Optional)** What is the total energy difference between the O$_2$ singlet and triplet state?
+===== Appendix =====
+==== Basis Set for Oxygen ====
+<code>
+#O  pc-1
+  0  0  6  2
+.70000000          0.00539400          0.00000000
+.15000000          0.04024800         -0.00031692
+.89000000          0.17921000         -0.00259440
+.87600000          0.45978000         -0.03624100
+.66460000          0.45234000         -0.08779000
+.06690000          0.00000000          0.53320000
+  0  0  1  1
+.30700000          1.00000000
+  1  1  3  1
+.02200000          0.04891900
+.68380000          0.24962000
+.99234000          0.51347000
+  1  1  1  1
+.24487000          1.00000000
+  2  2  1  1
+.00000000          1.00000000
+</code>
+==== Potential for Oxygen ====
+<code>
+#O ALLELECTRON ALL
+    4    0
+.24762086    0
+</code>
+==== Coordinates for O$_2$ ====
+<code>
+O         4.4720538104        4.7584649515        4.9999999998
+O         5.5279461896        5.2415350485        4.9999999995
+</code>
+==== Coordinates for CO$_2$ ====
+<code>
+  C         4.9999776408        4.9999662056        4.9999894728
+  O         5.6486993295        5.9339540261        5.0004691016
+  O         4.3512530072        4.0659797648        4.9995464311
+</code>
+==== Coordinates for H$_2$O ====
+<code>
+  O         4.6926974603        4.7525411835        4.6307067609
+  H         5.6350172910        4.8022721035        4.7052454388
+  H         4.3528571397        5.2445222023        5.3644975249
+</code>