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< CENTER > < A HREF = "http://lammps.sandia.gov" > LAMMPS WWW Site< / A > - < A HREF = "Manual.html" > LAMMPS Documentation< / A > - < A HREF = "Section_commands.html#comm" > LAMMPS Commands< / A >
< / CENTER >
< HR >
< H3 > kspace_style command
< / H3 >
< P > < B > Syntax:< / B >
< / P >
< PRE > kspace_style style value
< / PRE >
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< UL > < LI > style = < I > none< / I > or < I > ewald< / I > or < I > pppm< / I > or < I > pppm/cg< / I > or < I > pppm/tip4p< / I > or < I > ewald/n< / I > or < I > pppm/gpu< / I > or < I > ewald/omp< / I > or < I > pppm/omp< / I > or < I > pppm/proxy< / I >
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< PRE > < I > none< / I > value = none
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< I > ewald< / I > value = accuracy
accuracy = desired relative error in forces
< I > pppm< / I > value = accuracy
accuracy = desired relative error in forces
< I > pppm/cg< / I > value = accuracy (smallq)
accuracy = desired relative error in forces
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smallq = cutoff for charges to be considered (optional) (charge units)
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< I > pppm/tip4p< / I > value = accuracy
accuracy = desired relative error in forces
< I > ewald/n< / I > value = accuracy
accuracy = desired relative error in forces
< I > pppm/gpu< / I > value = accuracy
accuracy = desired relative error in forces
< I > ewald/omp< / I > value = accuracy
accuracy = desired relative error in forces
< I > pppm/omp< / I > value = accuracy
accuracy = desired relative error in forces
< I > pppm/proxy< / I > value = accuracy
accuracy = desired relative error in forces
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< / PRE >
< / UL >
< P > < B > Examples:< / B >
< / P >
< PRE > kspace_style pppm 1.0e-4
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kspace_style pppm/cg 1.0e-5 1.0e-6
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kspace_style none
< / PRE >
< P > < B > Description:< / B >
< / P >
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< P > Define a K-space solver for LAMMPS to use each timestep to compute
long-range Coulombic interactions or long-range 1/r^N interactions.
When such a solver is used in conjunction with an appropriate pair
style, the cutoff for Coulombic or other 1/r^N interactions is
effectively infinite; each charge in the system interacts with charges
in an infinite array of periodic images of the simulation domain.
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< / P >
< P > The < I > ewald< / I > style performs a standard Ewald summation as described in
any solid-state physics text.
< / P >
< P > The < I > pppm< / I > style invokes a particle-particle particle-mesh solver
< A HREF = "#Hockney" > (Hockney)< / A > which maps atom charge to a 3d mesh, uses 3d FFTs
to solve Poisson's equation on the mesh, then interpolates electric
fields on the mesh points back to the atoms. It is closely related to
the particle-mesh Ewald technique (PME) < A HREF = "#Darden" > (Darden)< / A > used in
AMBER and CHARMM. The cost of traditional Ewald summation scales as
N^(3/2) where N is the number of atoms in the system. The PPPM solver
scales as Nlog(N) due to the FFTs, so it is almost always a faster
choice < A HREF = "#Pollock" > (Pollock)< / A > .
< / P >
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< P > The < I > pppm/cg< / I > style is identical to the < I > pppm< / I > style except that it
has an optimization for systems where most particles are uncharged.
The optional < I > smallq< / I > argument defines the cutoff for the absolute
charge value which determines whether a particle is considered charged
or not. Its default value is 1.0e-5.
< / P >
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< P > The < I > pppm/tip4p< / I > style is identical to the < I > pppm< / I > style except that it
adds a charge at the massless 4th site in each TIP4P water molecule.
It should be used with < A HREF = "pair_style.html" > pair styles< / A > with a
< I > long/tip4p< / I > in their style name.
< / P >
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< P > The < I > ewald/n< / I > style augments < I > ewald< / I > by adding long-range dispersion
sum capabilities for 1/r^N potentials and is useful for simulation of
interfaces < A HREF = "#Veld" > (Veld)< / A > . It also performs standard coulombic Ewald
summations, but in a more efficient manner than the < I > ewald< / I > style.
The 1/r^N capability means that Lennard-Jones or Buckingham potentials
can be used with < I > ewald/n< / I > without a cutoff, i.e. they become full
long-range potentials.
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< / P >
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< P > Currently, only the < I > ewald/n< / I > style can be used with non-orthogonal
(triclinic symmetry) simulation boxes.
< / P >
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< P > The < I > pppm/proxy< / I > style is a special variant for calculations
in hybrid OpenMP/MPI parallel mode. It is functionally equivalent
with < I > pppm< / I > , but it its force computation is being executed
as a single thread concurrently with a multi-threaded non-bonded
calculation for a pair style with < I > pppm/omp< / I > suffix. For calcuations
across many multi-core nodes, this can have a performance benefit
over performing the real and reciprocal space part separately,
specifically when otherwise the time spent on the pair style
would slightly less than in < I > pppm< / I > without threading.
< / P >
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< P > Note that the PPPM styles can be used with single-precision FFTs by
using the compiler switch -DFFT_SINGLE for the FFT_INC setting in your
lo-level Makefile. This setting also changes some of the PPPM
operations (e.g. mapping charge to mesh and interpolating electric
fields to particles) to be performed in single precision. This option
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can speed-up long-range calulations, particularly in parallel or on
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GPUs. The use of the -DFFT_SINGLE flag is discussed in < A HREF = "Section_start.html#start_2_4" > this
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section< / A > of the manual.
< / P >
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< HR >
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< P > When a kspace style is used, a pair style that includes the
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short-range correction to the pairwise Coulombic or other 1/r^N forces
must also be selected. For Coulombic interactions, these styles are
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ones that have a < I > coul/long< / I > in their style name. For 1/r^6
dispersion forces in a Lennard-Jones or Buckingham potential, see the
< A HREF = "pair_lj_coul.html" > pair_style lj/coul< / A > or < A HREF = "pair_buck_coul.html" > pair_style
buck/coul< / A > commands.
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< / P >
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< P > The specified < I > accuracy< / I > determines the relative RMS error in per-atom
forces calculated by the long-range solver. It is set as a
dimensionless number, relative to the force that two unit point
charges (e.g. 2 monovalent ions) exert on each other at a distance of
1 Angstrom. This reference value was chosen as representative of the
magnitude of electrostatic forces in atomic systems. Thus an accuracy
value of 1.0e-4 means that the RMS error will be a factor of 10000
smaller than the reference force.
< / P >
< P > The accuracy setting is used in conjunction with the pairwise cutoff
to determine the number of K-space vectors for style < I > ewald< / I > or the
FFT grid size for style < I > pppm< / I > .
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< / P >
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< P > See the < A HREF = "kspace_modify.html" > kspace_modify< / A > command for additional
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options of the K-space solvers that can be set, including a < I > force< / I >
option for setting an absoulte RMS error in forces, as opposed to a
relative RMS error.
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< / P >
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< HR >
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< P > Styles with a < I > cuda< / I > , < I > gpu< / I > , < I > omp< / I > , or < I > opt< / I > suffix are functionally
the same as the corresponding style without the suffix. They have
been optimized to run faster, depending on your available hardware, as
discussed in < A HREF = "Section_accelerate.html" > Section_accelerate< / A > of the
manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
< / P >
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< P > More specifically, the < I > pppm/gpu< / I > style performs charge assignment and
force interpolation calculations on the GPU. These processes are
performed either in single or double precision, depending on whether
the -DFFT_SINGLE setting was specified in your lo-level Makefile, as
discussed above. The FFTs themselves are still calculated on the CPU.
If < I > pppm/gpu< / I > is used with a GPU-enabled pair style, part of the PPPM
calculation can be performed concurrently on the GPU while other
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calculations for non-bonded and bonded force calculation are performed
on the CPU.
< / P >
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< P > These accelerated styles are part of the USER-CUDA, GPU, USER-OMP, and
OPT packages respectively. They are only enabled if LAMMPS was built
with those packages. See the < A HREF = "Section_start.html#start_3" > Making LAMMPS< / A >
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section for more info.
< / P >
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< P > See < A HREF = "Section_accelerate.html" > Section_accelerate< / A > of the manual for
more instructions on how to use the accelerated styles effectively.
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< / P >
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< P > < B > Restrictions:< / B >
< / P >
< P > A simulation must be 3d and periodic in all dimensions to use an Ewald
or PPPM solver. The only exception is if the slab option is set with
< A HREF = "kspace_modify.html" > kspace_modify< / A > , in which case the xy dimensions
must be periodic and the z dimension must be non-periodic.
< / P >
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< P > Kspace styles are part of the KSPACE package. They are only enabled
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if LAMMPS was built with that package. See the < A HREF = "Section_start.html#start_3" > Making
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LAMMPS< / A > section for more info.
< / P >
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< P > The < I > ewald/n< / I > style is part of the USER-EWALDN package. It is only
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enabled if LAMMPS was built with that package. See the < A HREF = "Section_start.html#start_3" > Making
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LAMMPS< / A > section for more info.
< / P >
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< P > When using a long-range pairwise TIP4P potential, you must use kspace
style < I > pppm/tip4p< / I > and vice versa.
< / P >
< P > < B > Related commands:< / B >
< / P >
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< P > < A HREF = "kspace_modify.html" > kspace_modify< / A > , < A HREF = "pair_lj.html" > pair_style
lj/cut/coul/long< / A > , < A HREF = "pair_charmm.html" > pair_style
lj/charmm/coul/long< / A > , < A HREF = "pair_lj_coul.html" > pair_style
lj/coul< / A > , < A HREF = "pair_buck.html" > pair_style buck/coul/long< / A >
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< / P >
< P > < B > Default:< / B >
< / P >
< PRE > kspace_style none
< / PRE >
< HR >
< A NAME = "Darden" > < / A >
< P > < B > (Darden)< / B > Darden, York, Pedersen, J Chem Phys, 98, 10089 (1993).
< / P >
< A NAME = "Hockney" > < / A >
< P > < B > (Hockney)< / B > Hockney and Eastwood, Computer Simulation Using Particles,
Adam Hilger, NY (1989).
< / P >
< A NAME = "Pollock" > < / A >
< P > < B > (Pollock)< / B > Pollock and Glosli, Comp Phys Comm, 95, 93 (1996).
< / P >
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< A NAME = "Veld" > < / A >
< P > < B > (Veld)< / B > In 't Veld, Ismail, Grest, J Chem Phys, in press (2007).
< / P >
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