the nuts and bolts of first-principles simulation durham, 6th-13th december 2001 4: the new castep...

The Nuts and Bolts of First-Principles Simulation

Durham, 6th-13th December 2001

4: The New CASTEP

CASTEP Developers’ Groupwith support from the ESF k Network

New CAmbridge

Serial

Total

Energy Package

An introduction…The Nuts and Bolts of First-Principles Simulation

Durham, 6th-13th December 2001

Lecture 4:

Nuts and Bolts 2001

Lecture 4: Introduction to New CASTEP

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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!

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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

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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.)

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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

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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.

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New CASTEP Features

Parallel, portable code Geometry Optimisation

BFGS Damped MD

Molecular Dynamics NVE NVT

Linear Response for Phonon spectra

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CASTEP Features Continued Transition state search

LST/QST Electronic properties

Optical Spectra DOS Population Analysis

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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

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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

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How to Use the Code: The Cell File! Example cell file for primitive SiC

%block LATTICE_ABC3.074500 3.074500 3.07450060.0 60.0 60.0

%endblock LATTICE_ABC

%block POSITIONS_FRACSi 0.0000 0.0000 0.0000C 0.2500 0.2500 0.2500

%endblock POSITIONS_FRAC

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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

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Controlling the Calculation:The Parameter FileTask : GeometryOptimization

XC_functional PW91

Basis_precision Precise

Electronic_minimiser CG

Elec_energy_tol = 0.000001 eV

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More Parameters…

Geom_method BFGS

Geom_force_tol : 0.01 hartree/bohr

Fix_occupancies = TRUE

Continuation = my_last_run.check

Energy_unit = kcal/mol

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Example Output

[Summary of cell and parameters input]

----------------------------------------------------------------------- <-- SCFSCF loop Energy Energy gain Timer <-- SCF per atom (sec) <-- SCF----------------------------------------------------------------------- <-- SCFInitial 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…]

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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

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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

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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

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Utility Modules

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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

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Fundamental Modules

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Example type: Wavefunctiontype, 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!

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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)

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Functional Modules

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Use of Fundamental Building Blocks (e.g. Calculating H)! Apply kinetic energy operator to wvfncall wave_kinetic_energy(wvfn,ek,H_wvfn)! Calculate the charge densitycall density_calculate(wvfn,occ,dens)! Calculate the local potentialcall locpot_calculate(dens,local_pot)! Apply the local potential to the wavefunctioncall pot_apply(local_pot,wvfn,wvfn_temp)! Add Vloc|psi> to kinetic energy contributioncall wave_add(wvfn_temp,H_wvfn)! Apply the non-local potential to the wavefunctioncall nlpot_apply(wvfn,…,wvfn_temp)! Add Vnl|psi> to get final H|psi>call wave_add(wvfn_temp,H_wvfn)

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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

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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)

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Timings for bad_trace

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Timings for good_trace

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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…

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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!

the nuts and bolts of first-principles simulation durham, 6th-13th december 2001 4: the new castep...

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