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--- exercises:2017_uzh_cp2k-tutorial:gw [2017/07/04 07:54] – [5. Cubic-scaling GW calculations] jwilhelm
+++ exercises:2017_uzh_cp2k-tutorial:gw [2022/10/31 14:33] (current) – oschuett
@@ Line 1: / Line 1: @@
+<note warning>This text is out of date. Please use [[howto:gw | this howto]] instead.</note>
 ====== GW method for computing electronic levels ======
@@ Line 172: / Line 174: @@
 For periodic GW calculations, a special correction scheme is necessary. A similar problem is appearing in Hartree-Fock calculations. In HF, an easy way out is given by the truncation of the Coulomb operator which works due to the convenient form of the HF equations. GW does not exhibit this convenient form and therefore, this truncation does not work for GW calculations. The theory why a correction is necessary and the correction scheme is described in [[doi>10.1103/PhysRevB.95.235123]] [7].
-The basis can be found in {{exercises:2017_uzh_cp2k-tutorial:LiH_BASIS_RI.tar ?direct&400 |}}. Then run the calculation as listed below. The computed gap of LiH with the input from below should be 5.95 eV which is still significantly off from the converged result of 4.7 eV from [[doi>10.1103/PhysRevB.95.235123]] [7]. Here, a basis set extrapolation (as shown above) and a larger supercell are necessary to get closer to the result of 4.7 eV. Please redo the calculation without the flag PERIODIC in the GW section and see that the resulting gap of 11.25 eV is much more off than the gap with correction.
+The basis can be found in {{exercises:2017_uzh_cp2k-tutorial:LiH_BASIS_RI.tar ?direct&400 |}}. Then run the calculation as listed below. The computed gap of LiH with the input from below should be 6.05 eV which is still significantly off from the converged result of 4.7 eV from [[doi>10.1103/PhysRevB.95.235123]] [7]. Here, a basis set extrapolation (as shown above) and a larger supercell are necessary to get closer to the result of 4.7 eV. Please redo the calculation without the flag PERIODIC in the GW section and see that the resulting gap of 11.25 eV is much more off than the gap with correction.
 <code - LiH_periodic.inp>
@@ Line 270: / Line 272: @@
 Cubic-scaling GW calculations could be a more efficient alternative for large systems. See below an exemplary input for one water molecule. Compare the results to the ones from Sec. 1. In general, small deviations (< 0.05 eV) for GW levels can be expected from cubic-scaling GW calculations compared to canonical GW calculations due to additional approximations in cubic-scaling GW.
-Please observe that the input below is much slower than the input for canonical GW. Therefore, it can be benefitial to run it with more processes. The benefitial scaling of cubic-scaling GW only pays off for large systems where it is more efficient as canonical GW calculations (rule of thumb: cubic-scaling GW can be more efficient for systems with more than 100 atoms if the filter parameters are well set).
+Please observe that the input below is much slower than the input for canonical GW. Therefore, it can be beneficial to run it with more MPI tasks. The beneficial scaling of cubic-scaling GW only pays off for large systems where it is more efficient as canonical GW calculations (rule of thumb: cubic-scaling GW can be more efficient for systems with more than 100 atoms if the filter parameters are well set).
 <code - H2O_GW100_cubic_scaling.inp>
@@ Line 277: / Line 279: @@
   &DFT
     ! retrieve basis set from the CP2K trunk version
-    BASIS_SET_FILE_NAME BASIS_def2_QZVP_ALL
+    BASIS_SET_FILE_NAME BASIS_def2_QZVP_RI_ALL
     POTENTIAL_FILE_NAME POTENTIAL
     &MGRID
@@ Line 316: / Line 318: @@
           ! in imag. time and frequency
           MINIMAX
-          ! If the HOMO-LUMO gap of the system is small, more
+          ! If the HOMO-LUMO gap of the system is small, 20
-          ! points for the time/frequency grid have to be used
+          ! points for the time/frequency grid should be used
           ! (flag RPA_NUM_QUAD_POINTS). The time and frequency grid
           ! are equally large. The maximum grid size is 20.
-          ! In principle, the grid size is a convergence parameter
+          ! For large-gap systems (as the water molecule), 12 points
-          ! but for many large-gap systems (as the water molecule),
+          ! should be sufficient
-          ! convergence of this grid is hard to achieve due to
-          ! numerical instabilities.
           RPA_NUM_QUAD_POINTS  12
           ! imaginary time flag enables cubic-scaling RPA or
@@ Line 329: / Line 329: @@
           IM_TIME
           &IM_TIME
+            ! EPS_FILTER_IM_TIME should be tuned for the specific
+            ! application: the computational cost strongly
+            ! depends on EPS_FILTER
             EPS_FILTER_IM_TIME 1.0E-12
-            ! for large systems, increase GROUP_SIZE_3C not
+            ! for large systems, increase GROUP_SIZE_3C
-            ! to run out of memory (OOM)
+            ! to prevent out of memory (OOM)
             GROUP_SIZE_3C 1
             ! for extremely large systems, increase GROUP_SIZE_P
-            ! not to run OOM
+            ! to prevent OOM
             ! for very large systems, it is also recommended
-            ! to use OMP threads not to run OOM
+            ! to use OMP threads to prevent OOM
             GROUP_SIZE_P 1
+            ! for larger systems, MEMORY_CUT must be increased
+            ! to prevent out of memory (OOM)
+            MEMORY_CUT  1
             GW
           &END IM_TIME