13.7 The Second-Generation ALMO-EDA Method

13.7.5 Visualization Tools in EDA2

The following visualization tools are enabled in EDA2:

  • Automated generation of complementary occupied-virtual pairs (COVP)

  • Electron density difference (EDD) maps between intermediate states (FRZPOL, POLFull)

  • Plots for Natural Orbitals for Chemical Valence (NOCV)

As introduced in Sec. 13.5.2, the COVPs can help elucidate the details of a charge-transfer process by showing the chemically most relevant donor-acceptor orbitals. In its implementation in EDA2, we enabled an automated selection of the most significant occupied-virtual pairs (based on a threshold on singular values). The MO cube files of these selected COVPs are then generated, and thus there is no need to specify which orbitals to plot. This new feature can be turned on by setting EDA_COVP_PRINT = 2. Also, both the old and new formats of the $plots section are supported for automated COVP cube generation in EDA2. The old format requires MAKE_CUBE_FILES = TRUE and the new format requires PLOTS = TRUE.

EDA2 also enabled electron density difference (EDD) plots to show the redistribution of electron density upon polarization (Δ𝐏pol=𝐏pol-𝐏frz) and charge transfer (Δ𝐏ct=𝐏full-𝐏pol). Another related quantity that can be visualized is the so-called natural orbitals for chemical valence (NOCV), 641 which are defined as the eigenvectors of Δ𝐏=𝐏full-𝐏frz. The NOCVs appear in pairs ψk and ψ-k, whose associated eigenvalues are nk and -nk, respectively. The energy lowering associated with each pair of NOCVs can be calculated using the extended transition state (ETS) approach (see ref. 641 for details). The NOCVs are useful tools for illustrating the underlying orbital interactions in chemical bonding.

In EDA2, the EDD maps are plotted when EDA_PLOT_DIFF_DEN = TRUE. The calculation of NOCVs can be turned on by EDA_NOCV = TRUE, and the most significant NOCVs are automatically selected based on a threshold on the eigenvalues {nk} and then plotted. Note that the new format of the $plots section is required for these visualizations.

EDA_PLOT_DIFF_DEN
       Plot changes in electron density due to POL and CT
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       FALSE Do not make EDD plots TRUE Make EDD plots
RECOMMENDATION:
       None

EDA_NOCV
       Perform the NOCV analysis and plot the significant NOCVs
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       FALSE Do not perform NOCV analysis TRUE Perform NOCV analysis
RECOMMENDATION:
       None

Example 13.17  Performing perturbative CTA through EDA2 (using the default settings). The automatic COVP generation is enabled by setting EDA_PRINT_COVP = 2, which plots the most significant COVPs automatically. The new format of the $plots section is used by setting PLOTS = TRUE, and there is no need to specify how many orbitals to plot.

$molecule
0 1
--
0 1
O          -1.521720    0.129941    0.000000
H          -1.924536   -0.737533    0.000000
H          -0.571766   -0.039961    0.000000
--
0 1
O           1.362840   -0.099704    0.000000
H           1.727645    0.357101   -0.759281
H           1.727645    0.357101    0.759281
$end

$rem
   JOBTYPE               EDA
   METHOD                B3LYP
   BASIS                 6-31+G(d)
   SYMMETRY              FALSE
   SYM_IGNORE            FALSE
   MEM_TOTAL             8000
   MEM_STATIC            2000
   BASIS_LIN_DEP_THRESH  6
   THRESH                14
   SCF_CONVERGENCE       8
   MAXSCF                200
   EDA_COVP              TRUE
   EDA_PRINT_COVP        2  !auto-generation of covp cube files
   MAKE_CUBE_FILES       TRUE
   PLOTS TRUE            !new format for the plot section
$end

$plots
grid_points 100 100 100
$end

Example 13.18  Electron density difference (EDD) plots and NOCV analysis for the NH3-BH3 complex

$molecule
0 1
--
0 1
N         0.0000001517    0.7279666667    0.0000000000
H         0.9488005016    1.0881357449    0.0000000000
H        -0.4743994984    1.0881371276   -0.8216800000
H        -0.4743994984    1.0881371276    0.8216800000
--
0 1
B        -0.0000014567   -0.9275533333    0.0000000000
H        -1.1719117610   -1.2408021948    0.0000000000
H         0.5859582390   -1.2408039026   -1.0149100000
H         0.5859582390   -1.2408039026    1.0149100000
$end

$rem
   JOBTYPE                 eda
   EDA2                    2   !ALMO-POL
   METHOD                  b3lyp
   BASIS                   6-31g(d)
   SCF_ALGORITHM           diis
   XC_GRID                 1
   SCF_CONVERGENCE         8
   MAX_SCF_CYCLES          200
   THRESH                  14
   SYMMETRY                false
   SYM_IGNORE              true
   EDA_PLOT_DIFF_DEN TRUE  !plot EDD maps
   EDA_NOCV TRUE           !plot NOCVs
$end

$plots
grid_points 100 100 100
$end