1 / 32

The Nuts and Bolts of First-Principles Simulation

The Nuts and Bolts of First-Principles Simulation. 4: The New CASTEP. Durham, 6th-13th December 2001. CASTEP Developers’ Group with support from the ESF  k Network. Lecture 4:. New CA mbridge S erial T otal E nergy P ackage. An introduction….

evita
Download Presentation

The Nuts and Bolts of First-Principles Simulation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Nuts and Bolts of First-Principles Simulation 4: The New CASTEP Durham, 6th-13th December 2001 CASTEP Developers’ Groupwith support from the ESF k Network

  2. Lecture 4: NewCAmbridgeSerialTotalEnergyPackage An introduction… The Nuts and Bolts of First-Principles Simulation Durham, 6th-13th December 2001

  3. What New CASTEP is not • It’s not just CAmbridge • It’s not just Serial • It’s doesn’t just calculate Total Energies • However, it is still a Package! Lecture 4: Introduction to New CASTEP

  4. So, What is it Then? • Introduction to the CASTEP Developer’s Group • The goals of the New CASTEP project • A brief history of New CASTEP • The features of the package • How to use the code (command line) • Philosophy and design of the code • The future of New CASTEP Lecture 4: Introduction to New CASTEP

  5. The CASTEP Developers Group • Mike Payne (Cambridge) • Matt Probert (York) • Chris Pickard (Cambridge) • Stewart Clark (Durham) • Phil Hasnip (Cambridge) • Phil Lindan (Kent at Canterbury) • Matt Segall (Cambridge, Camitro UK Ltd.) Lecture 4: Introduction to New CASTEP

  6. The Goals of New CASTEP • Ease of maintenance • Ease of future development • Portability • Efficiency These goals are sometimes conflicting, therefore a compromise must be found Lecture 4: Introduction to New CASTEP

  7. A Brief History of New CASTEP • July ‘99: A meeting of interested parties. Is there a need for a new PW pseudopotential code? • July ’99 -> Jan. ’00: Informal specification group outlines the spec. of a new code. • Feb. ’00: Implementation of new code begins. CDG formed. • May ’01: Agreement reached with MSI (Now Accelrys) to commercialise New CASTEP. • Dec. ’01: Code freeze on first version of New CASTEP. Lecture 4: Introduction to New CASTEP

  8. New CASTEP Features • Parallel, portable code • Geometry Optimisation • BFGS • Damped MD • Molecular Dynamics • NVE • NVT • Linear Response for Phonon spectra Lecture 4: Introduction to New CASTEP

  9. CASTEP Features Continued • Transition state search • LST/QST • Electronic properties • Optical Spectra • DOS • Population Analysis Lecture 4: Introduction to New CASTEP

  10. CASTEP Technical Features • Ultrasoft/Norm-conserving pseudopotentials • Pseudopotential generation on-the-fly • Double grid technique for charge/potential grid • Multiple electronic minimisers • All-bands CG • RMM/DIIS Lecture 4: Introduction to New CASTEP

  11. Technical Features Cont… • Metals treated by • Density mixing • Ensemble DFT • Exchange-Correlation Potentials • LDA • Perdew-Wang `91 GGA • PBE/RPBE GGA • Non-linear core corrections Lecture 4: Introduction to New CASTEP

  12. How to Use the Code: The Cell File ! Example cell file for primitive SiC %block LATTICE_ABC 3.074500 3.074500 3.074500 60.0 60.0 60.0 %endblock LATTICE_ABC %block POSITIONS_FRAC Si 0.0000 0.0000 0.0000 C 0.2500 0.2500 0.2500 %endblock POSITIONS_FRAC Lecture 4: Introduction to New CASTEP

  13. More Cell File Keywords KPOINTS_MP_GRID 4 4 4 %block SYMMETRY_OPS … %endblock SYMMETRY_OPS %block SPECIES_POT C my_C_pot.usp %endblock SPECIES_POT Lecture 4: Introduction to New CASTEP

  14. Controlling the Calculation:The Parameter File Task : GeometryOptimization XC_functional PW91 Basis_precision Precise Electronic_minimiser CG Elec_energy_tol = 0.000001 eV Lecture 4: Introduction to New CASTEP

  15. More Parameters… Geom_method BFGS Geom_force_tol : 0.01 hartree/bohr Fix_occupancies = TRUE Continuation = my_last_run.check Energy_unit = kcal/mol Lecture 4: Introduction to New CASTEP

  16. Example Output [Summary of cell and parameters input] ----------------------------------------------------------------------- <-- SCF SCF loop Energy Energy gain Timer <-- SCF per atom (sec) <-- SCF ----------------------------------------------------------------------- <-- SCF Initial 6.99675047E+002 4.22 <-- SCF 1 -8.56371636E+002 1.94505835E+002 23.70 <-- SCF 2 -8.57260747E+002 1.11138925E-001 42.59 <-- SCF 3 -8.57286394E+002 3.20580434E-003 62.00 <-- SCF 4 -8.57286462E+002 8.55207285E-006 80.91 <-- SCF 5 -8.57286463E+002 3.12287085E-008 98.20 <-- SCF ----------------------------------------------------------------------- <-- SCF [Forces, stresses, Gemoetry optimisation, MD run…] Lecture 4: Introduction to New CASTEP

  17. Design Philosophy of New CASTEP • Written specification, prior to coding! • Implemented in FORTRAN90 • Modular approach (F90 not truly object oriented) • Data abstraction using derived data types • Overloading for simple, clear subroutine names Lecture 4: Introduction to New CASTEP

  18. Why FORTRAN90? • A language for numerical codes • ‘Designed’ with optimisation in mind  good numerical efficiency • ‘Modern’ high level language • Modules (classes) • Interfaces (prototypes) • Memory allocation • Derived types Lecture 4: Introduction to New CASTEP

  19. Code Structure (Overview) • Functional Modules • The ‘Physics’ • High-level • Fundamental Modules • The building blocks • Define types and operations • Utility Modules • Machine-dependant • Low-level algorithms Lecture 4: Introduction to New CASTEP

  20. Utility Modules Lecture 4: Introduction to New CASTEP

  21. Utility modules provide: • Fundamental constants • API for parallel execution • I/O • Basic operations, e.g. opening and closing files • Freeform file I/O • Generic algorithms • FFTs • Matrix inversion • Random numbers Lecture 4: Introduction to New CASTEP

  22. Fundamental Modules Lecture 4: Introduction to New CASTEP

  23. Example type: Wavefunction type, public :: wavefunction integer :: nbands ! Max no. bands integer :: nkpts ! No. of kpts integer :: nspins ! No. of spins represented complex(kind=dp), dimension(:,:,:,:), pointer :: coeffs logical :: paged ! Is the wavefn paged to disc? ! Iff paged:- integer :: page_unit ! the unit no. of the pagefile ... end type wavefunction N.B. This has been simplified for illustrative purposes! Lecture 4: Introduction to New CASTEP

  24. Example Operations: wavefunction_allocate(wvfn) wavefunction_deallocate(wvfn) wavefunction_initialise(wvfn,method) wavefunction_copy(wvfn1,wvfn2) wavefunction_add(wvfn2,wvfn2,wvfn_out) wavefunction_dot(wvfn1,wvfn2,products) wavefunction_orthogonalise(wvfn) wavefunction_recip_to_real(wvfn,nb,nk,ns,real_bnd) Overloading: wavefunction_add(wvfn1,wvfn2,wvfn_out,c1,c2) wavefunction_add(bnd,wvfn,nb,nk,ns) Lecture 4: Introduction to New CASTEP

  25. Functional Modules Lecture 4: Introduction to New CASTEP

  26. Use of Fundamental Building Blocks (e.g. Calculating H) ! Apply kinetic energy operator to wvfn call wave_kinetic_energy(wvfn,ek,H_wvfn) ! Calculate the charge density call density_calculate(wvfn,occ,dens) ! Calculate the local potential call locpot_calculate(dens,local_pot) ! Apply the local potential to the wavefunction call pot_apply(local_pot,wvfn,wvfn_temp) ! Add Vloc|psi> to kinetic energy contribution call wave_add(wvfn_temp,H_wvfn) ! Apply the non-local potential to the wavefunction call nlpot_apply(wvfn,…,wvfn_temp) ! Add Vnl|psi> to get final H|psi> call wave_add(wvfn_temp,H_wvfn) Lecture 4: Introduction to New CASTEP

  27. Coding Style • Clear code • Meaningful variable and subroutine names • Lots of comments • Good structure • Care with performance issues • Some features of FORTRAN90 sub-optimal • Use BLAS/LAPACK where applicable Lecture 4: Introduction to New CASTEP

  28. Example problem with F90 ! Finding the trace of a sub-array real, dimension(N,N) :: A ! Pass a sub-array of A of dimension m tr = bad_trace(A(1:m,1:m),m) !Pass the whole array with dimension of sub-array tr = good_trace(A,m,N) Lecture 4: Introduction to New CASTEP

  29. Timings for bad_trace Lecture 4: Introduction to New CASTEP

  30. Timings for good_trace Lecture 4: Introduction to New CASTEP

  31. The Future of New CASTEP • NPT Molecular Dynamics • Free-energy integration for reaction barriers • New non-local XC functionals • NMR chemical shifts • EELS • Raman spectroscopy • Self-consistent pseudopotentials • Etc… Lecture 4: Introduction to New CASTEP

  32. Summary • New CASTEP code designed and implemented by an academic group • Sound software design • Modern implementation • State-of-the-art features • Good performance • The future is bright! Lecture 4: Introduction to New CASTEP

More Related