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# VCS-MD with pwscf

VCS-MD with pwscf. &amp;CONTROL calculation = {&quot;vc-relax ” | ” vc-md ” } , nstep = 45 , etot_conv_thr = 1.0D-5 , forc_conv_thr = 1.0D-4 , dt = 100 , / &amp;CELL cell_dynamics = {'damp-w ’ |'damp-pr ’ |'w ’ | ’ pr ’ } , press = 0.00 ,

## VCS-MD with pwscf

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1. VCS-MD with pwscf

2. &CONTROL calculation = {"vc-relax”|”vc-md”} , nstep = 45 , etot_conv_thr = 1.0D-5 , forc_conv_thr = 1.0D-4 , dt = 100 , / &CELL cell_dynamics = {'damp-w’|'damp-pr’|'w’|’pr’} , press = 0.00 , wmass = 0.00150000 , / # A small convergence threshold for selfconsistency is # recommended: &ELECTRONS conv_thr = 1.0D-8 / • A few variables must be set

3. A7 to sc transition in As • Unit Cell: (a=b=c, cosAB=cosAC=cosBC) • Guessing a=3.85 Å, x=0.275, cosAB=0.49517470 • Energy Cut-Off=30 Ry. • 2 As per unit cell • 2As at ±(x, x, x,); • When x=0.25, cosAB=0.5 => Simple Cubic

4. Example 1 • Download (from Vlab web site) and unpack the file VCS.tar • Study the input files As0 (0 Gpa) and As40 (40 Gpa) • Run the examples • What is the final value of x at 0 GPa? And at 40 GPa? • What are the angles between vectors?

5. Stishovite SiO2 • Same structure as Rutile (TiO2) • Tetragonal (a=b≠c) • Try a=4.59, c=2.96 Å • Energy Cut-Off=30 Ry for US-pp, 70 Ry for NC-pp. • 2 SiO2 per unit cell • 2Si at (0,0,0); (.5,.5,.5) • 4O at ±(x,x,0); ±(.5+x,.5-x,.5) • x=0.3

6. Example 2 • Study the input file St-SiO2 (Stishovite SiO2 Input) • Run the example • What are the final values of x and (c/a)? • Try the pseudo-potential file O.rw2 for O. Wave function cut-off must be set to 70 Ry. Change charge density cut-off accordingly.

7. Home Work 1: Quartz • Hexagonal structure • Try (c/a)=1.1, a=4.72 Å • Energy Cut-Off=30 Ry for US-pp, 70 Ry for NC-pp. • 3 SiO2 per unit cell

8. Home Work 1- continued … • Prepare the input file • Atomic positions in crystal coordinates are given (aQtz-SiO2) • Optimize the crystal structure at 0 Gpa, using both pesudo-potentials available for O. • What are the final positions and value of (c/a)?

9. Home Work 2: MgSiO3 Ilmenite • Tetragonal Structure. Cell parameters and atomic positions are given in file Il_MgSiO3 • Complete the input file • Optimize the crystal structure at 0, 10, 20 and 30 GPa. • Plot the curve VxP (V=cell volume). • Try to produce a 3D vizualization of the structure.

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