MP2¶
Overview¶
There are (at least) three algorithms within NWChem that compute the MøllerPlesset (or manybody) perturbation theory secondorder correction^{1} to the HartreeFock energy (MP2). They vary in capability, the size of system that can be treated and use of other approximations
 Semidirect – this is recommended for most large applications (up to about 2800 basis functions), especially on the IBM SP and other machines with significant disk I/O capability. Partially transformed integrals are stored on disk, multipassing as necessary. RHF and UHF references may be treated including computation of analytic derivatives. This is selected by specifying mp2 on the task directive, e.g.
TASK MP2
 Fullydirect^{2} – this is of utility if only limited I/O
resources are available (up to about 2800 functions). Only RHF
references and energies are available. This is selected by
specifying
direct_mp2
on the task directive, e.g.
TASK DIRECT_MP2
 Resolution of the identity (RI) approximation MP2 (RIMP2)^{3} – this uses the RI approximation and is therefore only exact in the limit of a complete fitting basis. However, with some care, high accuracy may be obtained with relatively modest fitting basis sets. An RIMP2 calculation can cost over 40 times less than the corresponding exact MP2 calculation. RHF and UHF references with only energies are available. This is selected by specifying rimp2 on the task directive, e.g.,
TASK RIMP2
All three MP2 tasks share the same input block.
MP2
[FREEZE [[core] (atomic  <integer nfzc default 0>)] \
[virtual <integer nfzv default 0>]]
[TIGHT]
[PRINT]
[NOPRINT]
[VECTORS <string filename default scfoutputvectors> \
[swap [(alphabeta)] <integer pairlist>] ]
[RIAPPROX <string riapprox default V>]
[FILE3C <string filename default $file_prefix$.mo3cint>
[SCRATCHDISK <integer>]
END
FREEZE: Freezing orbitals¶
All MP2 modules support frozen core orbitals, however, only the direct MP2 and RIMP2 modules support frozen virtual orbitals.
By default, no orbitals are frozen. The atomic keyword causes orbitals to be frozen according to the rules in the table below. Note that no orbitals are frozen on atoms on which the nuclear charge has been modified either by the user or due to the presence of an ECP. The actual input would be
freeze atomic
For example, in a calculation on Si(OH)_{2}, by default the lowest seven orbitals would be frozen (the oxygen 1s, and the silicon 1s, 2s and 2p).
Number of orbitals considered “core” in the “freeze by atoms” algorithm¶
Period  Elements  Core Orbitals  Number of Core 

0  H  He    0 
1  Li  Ne  1s  1 
2  Na  Ar  1s2s2p  5 
3  K  Kr  1s2s2p3s3p  9 
4  Rb  Xe  1s2s2p3s3p4s3d4p  18 
5  Cs  Rn  1s2s2p3s3p4s3d4p5s4d5p  27 
6  Fr  Lr  1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p  43 
Caution: The rule for freezing orbitals “by atoms” are rather unsophisticated since the number of orbitals to be frozen is computed from the table above by summing the number of core orbitals in each atom present. Therefore, the corresponding number of lowestenergy orbitals are frozen. If for some reason the actual core orbitals are not the lowest lying, then correct results will not be obtained. It is likely that special attention should be paid to systems including third and higher period atoms.
The user may also specify the number of orbitals to be frozen by atom. Following the Si(OH)_{2} example, the user could specify
freeze atomic O 1 Si 3
In this case only the lowest four orbitals would be frozen. If the user does not specify the orbitals by atom, the rules default to values reported in the Table above.
Caution: The system does not check for a valid number of orbitals per atom. If the user specifies to freeze more orbitals then are available for the atom, the system will not catch the error. The user must specify a logical number of orbitals to be frozen for the atom.
The FREEZE directive may also be used to specify the number of core orbitals to freeze. For instance, to freeze the first 10 orbitals
freeze 10
or equivalently, using the optional keyword core
freeze core 10
Again, note that if the 10 orbitals to be frozen do not correspond to the first 10 orbitals, then the swap keyword of the VECTORS directive must be used to order the input orbitals correctly (MO vectors).
To freeze the highest virtual orbitals, use the virtual keyword. For instance, to freeze the top 5 virtuals
freeze virtual 5
Again, note that this only works for the directMP2 and RIMP2 energy codes.
TIGHT: Increased precision¶
The TIGHT directive can be used to increase the precision in the MP2 energy and gradients.
By default the MP2 gradient package should compute energies accurate to better than a microHartree, and gradients accurate to about five decimal places (atomic units). However, if there is significant linear dependence in the basis set the precision might not be this good. Also, for computing very accurate geometries or numerical frequencies, greater precision may be desirable.
This option increases the precision to which both the SCF (from 10^{6} to 10^{8} and CPHF 10^{4} to $10^{6} are solved, and also tightens thresholds for computation of the AO and MO integrals (from 10^{9} to 10^{11} within the MP2 code.
SCRATCHDISK: Limiting I/O usage¶
This directive  used only in the semidirect algorithm  allows to limit the per process disk usage. Mandatory argument for this keyword is the maximum number of MBytes. For example, the following input line
scratchdisk 512
puts an upper limit of 512 MBytes to the semidirect MP2 usage of disk (again, on a per process base).
PRINT and NOPRINT¶
The standard print control options are recognized. The list of recognized names are given in the table below.
Item  Print Level  Description 

RIMP2  
“2/3 ints”  debug  Partial 3center integrals 
“3c ints”  debug  MO 3center integrals 
“4c ints b”  debug  “B” matrix with approx. 4c integrals 
“4c ints”  debug  Approximate 4center integrals 
“amplitudes”  debug  “B” matrix with denominators 
“basis”  high  
“fit xf”  debug  Transformation for fitting basis 
“geombas”  debug  Detailed basis map info 
“geometry”  high  
“information”  low  General information about calc. 
“integral i/o”  high  File size information 
“mo ints”  debug  
“pair energies”  debug  (working only in direct_mp2 ) 
“partial pair energies”  debug  Pair energy matrix each time it is updated 
“progress reports”  default  Report completion of timeconsuming steps 
“reference”  high  Details about reference wavefunction 
“warnings”  low  Nonfatal warnings 
Printable items in the MP2 modules and their default print levels
VECTORS: MO vectors¶
All of the (supported) MP2 modules require use of converged canonical SCF (RHF or UHF) orbitals for correct results. The vectors are by default obtained from the preceding SCF calculation, but it is possible to specify a different source using the VECTORS directive. For instance, to obtain vectors from the file /tmp/h2o.movecs, use the directive
vectors /tmp/h2o.movecs
As noted above (FREEZE) if the SCF orbitals are not in the correct order, it is necessary to permute the input orbitals using the swap keyword of the VECTORS directive. For instance, if it is desired to freeze a total six orbitals corresponding to the SCF orbitals 15, and 7, it is necessary to swap orbital 7 into the 6th position. This is accomplished by
vectors swap 6 7
The swap capability is examined in more detail in Input/output of MO vectors.
RIMP2 fitting basis¶
The RIMP2 method requires a fitting basis, which must be specified with the name “rimp2 basis” (see Basis). For instance,
basis "rimp2 basis"
O s; 10000.0 1
O s; 1000.0 1
O s; 100.0 1
...
end
Alternatively, using a standard capability of basis sets (Basis) another named basis may be associated with the fitting basis. For instance, the following input specifies a basis with the name “small fitting basis” and then defines this to be the “rimp2 basis”.
basis "small fitting basis"
H s; 10 1
H s; 3 1
H s; 1 1
H s; 0.1 1
H s; 0.01 1
end
set "rimp2 basis" "small fitting basis"
FILE3C: RIMP2 3center integral filename¶
The default name for the file used to store the transformed 3center integrals is “file_prefix.mo3cint” in the scratch directory. This may be overridden using the FILE3C directive. For instance, to specify the file /scratch/h2o.3c, use this directive
file3c /scratch/h2o.3c
RIAPPROX: RIMP2 Approximation¶
The type of RI approximation used in the RIMP2 calculation is controlled by means of the RIAPPROX directive. The two possible values are V and SVS (case sensitive), which correspond to the approximations with the same names described by Vahtras et al.^{4}. The default is V.
Advanced options for RIMP2¶
These options, which functioned at the time of writing, are not currently supported.
Control of linear dependence¶
Construction of the RI fit requires the inversion of a matrix of fitting
basis integrals which is carried out via diagonalization. If the fitting
basis includes near linear dependencies, there will be small eigenvalues
which can ultimately lead to nonphysical RIMP2 correlation energies.
Eigenvectors of the fitting matrix are discarded if the corresponding
eigenvalue is less than min eval
which defaults to 10^{8}. This
parameter may be changed by setting the a parameter in the database. For
instance, to set it to 10^{10}
set "mp2:fit min eval" 1e10
Reference Spin Mapping for RIMP2 Calculations¶
The user has the option of specifying that the RIMP2 calculations are to be done with variations of the SCF reference wavefunction. This is accomplished with a SET directive of the form,
set "mp2:reference spin mapping" <integer array default 0>
Each element specified for array is the SCF spin case to be used for the corresponding spin case of the correlated calculation. The number of elements set determines the overall type of correlated calculation to be performed. The default is to use the unadulterated SCF reference wavefunction.
For example, to perform a spinunrestricted calculation (two elements) using the alpha spin orbitals (spin case 1) from the reference for both of the correlated reference spin cases, the SET directive would be as follows,
set "mp2:reference spin mapping" 1 1
The SCF calculation to produce the reference wavefunction could be either RHF or UHF in this case.
The SET directive for a similar case, but this time using the betaspin SCF orbitals for both correlated spin cases, is as follows,
set "mp2:reference spin mapping" 2 2
The SCF reference calculation must be UHF in this case.
The SET directive for a spinrestricted calculation (one element) from the betaspin SCF orbitals using this option is as follows,
set "mp2:reference spin mapping" 2
The SET directive for a spinunrestricted calculation with the spins flipped from the original SCF reference wavefunction is as follows,
set "mp2:reference spin mapping" 2 1
Batch Sizes for the RIMP2 Calculation¶
The user can control the size of each batch in the transformation and energy evaluation in the MP2 calculation, and consequently the memory requirements and number of passes required. This is done using two SET directives of the following form,
set "mp2:transformation batch size" <integer size default 1>
set "mp2:energy batch size" <integer isize jsize default 1 1>
The default is for the code to determine the batch size based on the available memory. Should there be problems with the programdetermined batch sizes, these variables allow the user to override them. The program will always use the smaller of the user’s value of these entries and the internally computed batch size.
The transformation batch size computed in the code is the number of
occupied orbitals in the (occ virfit) threecenter integrals to be
produced at a time. If this entry is less than the number of occupied
orbitals in the system, the transformation will require multiple passes
through the twoelectron integrals. The memory requirements of this
stage are two global arrays of dimension batch size
x vir x fit with
the “fit” dimension distributed across all processors (on shellblock
boundaries). The compromise here is memory space versus multiple
integral evaluations.
The energy evaluation batch sizes are computed in the code from the
number of occupied orbitals in the two sets of threecenter integrals to
be multiplied together to produce a matrix of approximate fourcenter
integrals. Two blocks of integrals of dimension (batch isize
x vir)
and (batch jsize
x vir) by fit are read in from disk and multiplied
together to produce batch isize
batch jsize
vir^2 approximate
integrals. The compromise here is performance of the distributed matrix
multiplication (which requires large matrices) versus memory space.
Energy Memory Allocation Mode: RIMP2 Calculation¶
The user must choose a strategy for the memory allocation in the energy evaluation phase of the RIMP2 calculation, either by minimizing the amount of I/O, or minimizing the amount of computation. This can be accomplished using a SET directive of the form,
set "mp2:energy mem minimize" <string mem_opt default I>
A value of I entered for the string mem_opt
means that a strategy to
minimize I/O will be employed. A value of C tells the code to use a
strategy that minimizes computation.
When the option to minimize I/O is selected, the block sizes are made as large as possible so that the total number of passes through the integral files is as small as possible. When the option to minimize computation is selected, the blocks are chosen as close to square as possible so that permutational symmetry in the energy evaluation can be used most effectively.
Local Memory Usage in ThreeCenter Transformation¶
For most applications, the code will be able to size the blocks without help from the user. Therefore, it is unlikely that users will have any reason to specify values for these entries except when doing very particular performance measurements.
The size of xf3ci:AO 1 batch size
is the most important of the three, in
terms of the effect on performance.
Local memory usage in the first two steps of the transformation is controlled in the RIMP2 calculation using the following SET directives,
set "xf3ci:AO 1 batch size" <integer max>
set "xf3ci:AO 2 batch size" <integer max>
set "xf3ci:fit batch size" <integer max>
The size of the local arrays determines the sizes of the two matrix multiplications. These entries set limits on the size of blocks to be used in each index. The listing above is in order of importance of the parameters to performance, with xf3ci:AO 1 batch size being most important.
Note that these entries are only upper bounds and that the program will size the blocks according to what it determines as the best usage of the available local memory. The absolute maximum for a block size is the number of functions in the AO basis, or the number of fitting basis functions on a node. The absolute minimum value for block size is the size of the largest shell in the appropriate basis. Batch size entries specified for max that are larger than these limits are automatically reset to an appropriate value.
Oneelectron properties and natural orbitals¶
If an MP2 energy gradient is computed, all contributions are available to form the MP2 linearresponse density. This is the density that when contracted with any spinfree, oneelectron operator yields the associated property defined as the derivative of the energy. Thus, the reported MP2 dipole moment is the derivative of the energy w.r.t. an external electric field and is not the expectation value of the operator over the wavefunction. It has been shown that evaluating the MP2 density through a derivative provides more accurate results, presumably because this matches the way experiments probe the electron density more closely[raghavachari1981]^{5}^{6}^{7}.
Only dipole moments are printed by the MP2 gradient code, but natural orbitals are produced and stored in the permanent directory with a file extension of “.mp2nos”. These may be fed into the property package to compute more general properties as in the following example.
start h2o
geometry
O 2.15950 0.88132 0.00000
H 3.12950 0.88132 0.00000
H 1.83617 0.89369 0.91444
end
basis spherical
* library augccpVDZ
end
mp2
freeze atomic
end
task mp2 gradient
property
vectors h2o.mp2nos
mulliken
end
task mp2 property
Note that the MP2 linear response density matrix is not necessarily positive definite so it is not unusual to see a few small negative natural orbital occupation numbers. Significant negative occupation numbers have been argued to be a sign that the system might be near degenerate^{8}.
SCSMP2: SpinComponent Scaled MP2¶
Each MP2 output contains the calculation of the SCSMP2 correlation energies as suggested by S.Grimme^{9}
The SCS keyword is only required for gradients calculations:
MP2
[SCS]
END
Scaling factors for the two components (parallel and opposite spin) can be defined by using the keywords FSS (same spin factor) and FOS (opposite spin factor):
mp2
scs
fss 1.13
fos 0.56
end
Default values are FSS=0.333333333, FOS=1.2 for MP2, and FSS=1.13, FOS=1.27 for CCSD.
References¶

Møller, Chr.; Plesset, M. S. Note on an Approximation Treatment for ManyElectron Systems. Physical Review 1934, 46 (7), 618–622. https://doi.org/10.1103/PhysRev.46.618. ↩

Wong, A. T.; Harrison, R. J.; Rendell, A. P. Parallel Direct FourIndex Transformations. Theoretica Chimica Acta 1996, 93 (6), 317–331. https://doi.org/10.1007/BF01129213. ↩

Bernholdt, D. E.; Harrison, R. J. LargeScale Correlated Electronic Structure Calculations: The RIMP2 Method on Parallel Computers. Chemical Physics Letters 1996, 250 (56), 477–484. https://doi.org/10.1016/00092614(96)000541. ↩

Vahtras, O.; Almlöf, J.; Feyereisen, M. W. Integral Approximations for LCAOSCF Calculations. Chemical Physics Letters 1993, 213 (56), 514–518. https://doi.org/10.1016/00092614(93)891517. ↩

Diercksen, G. H. F.; Roos, B. O.; Sadlej, A. J. Legitimate Calculation of FirstOrder Molecular Properties in the Case of Limited CI Functions. Dipole Moments. Chemical Physics 1981, 59 (12), 29–39. https://doi.org/10.1016/03010104(81)800821. ↩

Rice, J. E.; Amos, R. D. On the Efficient Evaluation of Analytic Energy Gradients. Chemical Physics Letters 1985, 122 (6), 585–590. https://doi.org/10.1016/00092614(85)872754. ↩

Wiberg, K. B.; Hadad, C. M.; LePage, T. J.; Breneman, C. M.; Frisch, M. J. Analysis of the Effect of Electron Correlation on Charge Density Distributions. The Journal of Physical Chemistry 1992, 96 (2), 671–679. https://doi.org/10.1021/j100181a030. ↩

Gordon, M. S.; Schmidt, M. W.; Chaban, G. M.; Glaesemann, K. R.; Stevens, W. J.; Gonzalez, C. A Natural Orbital Diagnostic for Multiconfigurational Character in Correlated Wave Functions. The Journal of Chemical Physics 1999, 110 (9), 4199–4207. https://doi.org/10.1063/1.478301. ↩

Grimme, S. Improved SecondOrder MøllerPlesset Perturbation Theory by Separate Scaling of Parallel and AntiparallelSpin Pair Correlation Energies. The Journal of Chemical Physics 2003, 118 (20), 9095–9102. https://doi.org/10.1063/1.1569242. ↩