Transcript The Nuts and Bolts of First

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 CAmbridge
Serial
Total
Energy
Package
An introduction…
The Nuts and Bolts of
First-Principles Simulation
Durham,
6th-13th December 2001
What New CASTEP is not
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It’s not just CAmbridge
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It’s not just
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It’s doesn’t just calculate Total Energies
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However, it is still a Package!
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Lecture 4: Introduction to New CASTEP
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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
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These goals are sometimes conflicting,
therefore a compromise must be found
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A Brief History of New
CASTEP
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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
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Molecular Dynamics
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BFGS
Damped MD
NVE
NVT
Linear Response for Phonon spectra
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CASTEP Features Continued
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Transition state search
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LST/QST
Electronic properties
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Optical Spectra
DOS
Population Analysis
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CASTEP Technical Features
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Ultrasoft/Norm-conserving pseudopotentials
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Pseudopotential generation on-the-fly
Double grid technique for charge/potential
grid
 Multiple electronic minimisers
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All-bands CG
RMM/DIIS
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Technical Features Cont…
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Metals treated by
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Exchange-Correlation Potentials
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Density mixing
Ensemble DFT
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_ABC
3.074500
3.074500
60.0
60.0
%endblock LATTICE_ABC
3.074500
60.0
%block POSITIONS_FRAC
Si 0.0000
0.0000
0.0000
C 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 File
Task : 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]
----------------------------------------------------------------------SCF loop
Energy
Energy gain
Timer
per atom
(sec)
----------------------------------------------------------------------Initial 6.99675047E+002
4.22
1 -8.56371636E+002
1.94505835E+002
23.70
2 -8.57260747E+002
1.11138925E-001
42.59
3 -8.57286394E+002
3.20580434E-003
62.00
4 -8.57286462E+002
8.55207285E-006
80.91
5 -8.57286463E+002
3.12287085E-008
98.20
-----------------------------------------------------------------------
<-<-<-<-<-<-<-<-<-<-<--
SCF
SCF
SCF
SCF
SCF
SCF
SCF
SCF
SCF
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
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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
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Basic operations, e.g. opening and closing files
Freeform file I/O
Generic algorithms
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FFTs
Matrix inversion
Random numbers
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Fundamental Modules
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Example type: Wavefunction
type, public :: wavefunction
integer :: nbands
integer :: nkpts
integer :: nspins
! Max no. bands
! No. of kpts
! No. of spins represented
complex(kind=dp), dimension(:,:,:,:), pointer :: coeffs
logical :: paged
integer :: page_unit
! Is the wavefn paged to disc?
! Iff paged:! 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 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)
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Coding Style
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Clear code
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Meaningful variable and subroutine names
Lots of comments
Good structure
Care with performance issues
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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!
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