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- A very powerful and easy-to-use RESP module is added into main function 7 (Population analysis). It can calculate the well-known ElectroStatic Potential (RESP) charge proposed in J. Phys. Chem., 97, 10269 (1993), and can also calculate ESP fitting charges under various customized conditions such as atomic equivalence constraint and fragment charge constraint. Multiple conformation is fully supported. See Section 3.9.16 for detailed introduction and Section 4.7.7 for examples. This module should be able to bring a revolution in the field of RESP charge derivation.
- Energy decomposition analysis based on UFF/AMBER/GAFF molecular forcefield is supported as subfunction 1 of main function 21. See corresponding introduction in Section 3.24.1 and examples in Section 4.21.1 of the manual.
- A new parameter "cubegenpath" is introduced into settings.ini file. If the parameter is set to actual path of cubegen utility of Gaussian package and the input file is .fch/fchk type, for most analyses of electrostatic potential (ESP), such as plotting plane map of ESP, molecular surface analysis of ESP, the ESP data will be calculated using cubegen instead of internal code of Multiwfn, the overall computational time will be significantly reduced, especially for large systems (since speed of calculating ESP by cubegen is evidently faster than current version of Multiwfn). See Section 5.7 of manual for detail.
- If .chg file is used as input file, now it can be converted to .pqr file using subfunction 2 of main function 100. The .pqr file can be directly loaded into VMD. This feature is very useful if you want to vividly exhibit atomic properties (e.g. atomic charges, atomic spin populations, condensed Fukui function) calculated by Multiwfn by means of coloring atoms. See Section 4.A.10 of the manual for illustration.
- Subfunction 21 of main function 100 is extended, now it can easily calculate molecular length/width/height and diameter. See Section 4.200.21 of the manual for example.
- Raman optical activity (ROA) spectrum now can be plotted via main function 11 based on Gaussian output file, see Section 3.13 for detail and Section 4.11.7 for example.
- Almost all kinds of kinetic energy density (more than twenty) have been supported by Multiwfn as user-defined function 1200. See corresponding part of Section 2.7 of the manual for detail.
- Option -3 is added to MK and CHELPG charges calculation module, by using it, it is able to examine electrostatic potential reproducibility of given atomic charges around the whole system or around specific region, see Section 4.7.8 for example.

- Hole-electron analysis module has been significantly rewritten. Definition of some indices have been changed and the result will be different to older version. This module now supports a new definition for measuring overlap between hole and electron, it is named as Sr, while the old one is named as Sm. Basis function, atom and fragment contribution to hole and electron distribution now can be directly printed. In addition, atom and fragment contribution can be vividly plotted as heat map. See Section 3.21.1 for introduction and Section 4.18.1 for example.
- By newly added subfunction 13 of main function 18, natural orbitals for a batch of selected excited states can be generated and exported to .molden file. See Section 3.21.13 for detail and Section 4.18.13 for example.
- The transition density matrix plotting function (subfunction 2 of main function 18) now can plot fragment based TDM map. In addition, this function now can automatically generate TDM between ground state and selected excited state and thus becomes much easier to use. See Section 3.21.2 for introduction and Section 4.18.2 for example.
- The function of generating transition density matrix (TDM) has been moved to subfunction 9 of main function 18. At the meantime, this function now supports generating TDM between two selected excited states.
- Definition of some quantites outputted by subfunction 3 of main function 18 (Analyze charge-transfer based on density difference grid data) has been modified to make the result more meaningful, see Section 3.21.3 for detail.
- Delta_r index now can be calculated for a batch of excited states at one time (subfunction 4 of main function 18).
- Speed of calculating transition electric dipole moment between excited states (subfunction 5 of main function 18) has been remarkably improved.
- In the function "Calculate interfragment charge transfer in electron excitation via IFCT method" (subfunction 8 of main function 18), a batch of fragments now can be simultaneously defined and the result between all fragments are outputted together. Notice that the equation used in this function in older version is incorrect, this problem has been fixed. See Section 3.21.8 for introduction of this method and Section 4.18.8 for example.
- The function "Generate transition density matrix" has been moved to subfunction 9 of main function 18 from hole-electron analysis module. At the meantime, speed of this function was significantly improved.
- The function "Decompose transition electric dipole moment as molecular orbital pairs contribution" has been moved to subfunction 10 of main function 18 from hole-electron analysis module. At the meantime, speed of this function was significantly improved, and the terms can be sorted and outputted according to contribution to transition dipole moment.
- The function "Decompose transition dipole moment as basis function and atom contributions" has been moved to subfunction 11 of main function 18 from hole-electron analysis module.
- The function "Check, modify and export configuration coefficients of an excitation" has been moved to subfunction -1 of main function 18 from hole-electron analysis module. In addition, this function now can export user-modified configuration coefficients to an external file, which can then be directly used as input file for all subfunctions in main function 18.
- Output files of TDDFT task of Firefly and GAMESS-US programs are fully supported as input file for all kinds of electron excitation analyses, see beginning of Section 3.21.

Numerous improvements and changes have been made for main function 18, they are summarized as follows. At the meantime, the corresponding sections of the manual have been significantly rewritten.

- Calculation speed of charge decomposition analysis for large systems has been significantly improved.
- Section 4.A.13 has been added to manual, it describes how to very easily plot pretty ESP colored molecular vdW surface map as well as penetration map of monomer vdW surface in VMD program based on output file of Multiwfn.
- Section 4.2.5 has been added to the manual, it describes how to very easily plot pretty AIM critical points and topology paths in VMD program based on output file of Multiwfn.
- In the MK and CHELPG module, if option 6 as been chosen once, then after calculation, fitting points with exact ESP value or absolute difference between the exact ESP and the ESP evaluated by atomic charges can be exported to .pqr file, which can be directly render in VMD. The example in Section 4.7.8 utilized this feature.
- Option -1 is added to the plotting plane definition interface of main function 4. By this option you can set translation and rotation of the plotting plane. This point has been mentioned in Section 3.5.2 of the manual, a practical instance of using this option was posted on http://bbs.keinsci.com/thread-11037-1-1.html.
- Algorithm detail of ADCH atomic charge has been changed, see Section 3.9.9 of the manual for detail. If the system has local planar (or almost planar) regions, the ADCH charges in these regions obtained via the new version may be different to those obtained via older versions. The result produced by the new version should be more reasonable. Similarly, the result of atomic dipole moment corrected Becke charges is also different to the older versions.
- .pqr file is supported as input file. For Multiwfn, the information provided by .pqr and .chg is the same, namely atom information as well as atomic charges, see Section 2.5 of the manual.
- Overband and combination band of IR, VCD and Raman spectra now can be plotted by main function 11 based on output file of corresponding Gaussian anharmonic tasks.
- Output file of Firefly has been experimentally supported. After changing the suffix of output file of Firefly to .gms, the file can be directly loaded into Multiwfn to provide wavefunction information.
- Molden input file produced by NWChem is formally supported. See beginning of Chapter 4 of the manual on how to properly generated it.
- Option 8 is added to post-process menu of main function 4 for most kinds of plane maps. Using this option, chemical bonds can be drawn on the graph as straight lines.
- When using Independent Gradient Model (IGM) anaylsis, if your input file contains wavefunction information, the program will let you choose the kind of the sign(lambda2)rho to be used, the first one is that based on actual electron density, the second one is that based on promolecular density.
- Section 4.7.6 is added to the manual, in which I discussed how to easily determine correspondence between basis functions and atomic orbitals via Mulliken population analysis.
- Section 4.4.9 is added to the manual to illustrate how to plot LOL-pi map for porphyrin to reveal favorable electron delocalization path.
- Section 4.2.4 is added to the manual to illustrate how to decompose properties at a critical point or given point as orbital contributions.
- Interface of Mulliken population analysis (MPA) is improved, meantime Section 4.7.0 is added to the manual to illustrate the use of MPA.
- When outputting vtx.pdb in post-process menu of quantitative molecular surface analysis, for electrostatic potential analysis, if value at any surface vertex exceeded recording limit of B-factor field of .pdb file, eV will be used instead of kcal/mol.
- Content of Section 4.12.7 of the manual has been replaced, now it corresponds to a newly added example, namely illustrating how to predict density of molecular crystal based on result of quantitative molecular surface analysis
- Section 4.17.1 of the manual is extended to illustrate how to carry out AIM basin analysis for the systems containing pseudoatoms (non-nuclear attractors of electron density)
- After integrating specific domain in domain analysis module (subfunction 14 of main function 200), minimum and maximum X/Y/Z of points belonging to the domain, as well as span distance in X/Y/Z will be outputted. In addition, option 11 is added to post-process menu, which is used to export boundary grids of specific domain to a .pdb file, so that you can easily use such as VMD program to measure size of the domain. These updates are quite useful for characterizing molecular cavity (see Section 4.200.14.2 of the manual)
- In the interface of defining fragments for plotting PDOS and OPDOS, the fragments now can be directly defined according to angular moment of atomic orbital. Meantime, the DOS plotting example in Section 4.10 has been extended to reflect this improvement
- Subfunction 28 is added to wavefunction modification module (main function 6), it is used to modify orbital energies. This function is useful when you want to rectify the orbital energies using a given relationship (e.g. J. Am. Chem. Soc., 121, 3414 (1999)) before plotting density-of-states (DOS) map.

- Fixed a fatal bug in the calculation of beta, gamma and delta via sum-of-states (SOS) method. This bug was introduced since version 3.5.
- Due to some bugs in EDFlib library, (3,+3) rather than (3,-3) type of AIM critical points are located at nuclear position for some elements when pseudopotential is employed. This problem has been fixed via updating EDFlib.
- When drawing spectra for multiple systems based on .dat file outputted by Grimme's sTDA program, Multiwfn crashes. This problem has been fixed.
- When custom operation involves "+" operator, the program doesn't work. This problem has been fixed, thanks jimkress for reporting.
- GAMESS-US output file cannot be loaded properly when pseudopotential is used, this problem has been solved, thanks PedroS for reporting.

- A new main function 20 is added, which is a collection of all visual study methods for weak interaction. It can carry out NCI (with/without promolecular approximation), aNCI, DORI, which have already been supported in eariler version, and the newly supported IGM analysis. Correspondingly, the manual is rearranged.
- The Independent Gradient Model (IGM) analysis method proposed in Phys. Chem. Chem. Phys., 19, 17928 (2017) is fully supported and can be performed via subfunction 10 of main function 20. This method can be used to individually visualize intra-fragment and inter-fragment interactions, the contribution of atomic pairs and atoms can be quantified and vividly rendered with help of VMD. In addition, the delta_g function involved in IGM analysis is added as the 22th real space function, its value at bond critical point in weak interaction region is shown to be closely related to interaction strength. See Section 3.22.5 of manual for introduction of IGM method, analysis examples are given in Section 4.20.10.
- Orbital localization analysis module (subfunction 13 of main function 200 in older version) is greatly improved and extended, now it is appeared as main function 19 since it is frequently used. Foster-Boys localization method and Pipek-Mezey localization with Lowdin population are added. In addition, the speed of Pipek-Mezey localization is signifcantly improved compared to older version, now it can be easily used for localizing occupuied orbitals for a system containing about 200 atoms. Moreoever, major character of resulting LMOs are automatically printed so that the users can quickly find the orbitals they are interested in.
- The total/dynamic/nondynamic electron correlation index proposed in Phys. Chem. Chem. Phys., 18, 24015 (2016) have been implemented as subfunction 15 of main function 200. Details can be found in Section 3.200.15 of the manual. .wfn/.wfx/.molden file carrying natural orbitals could be used as input file.
- CCSD(T) wavefunction generated by PSI4 program and arbitrary order of coupled-cluster and CI wavefunctions (including FCI) generated by Kallay's MRCC program can be analyzed by Multiwfn. See Section 4.A.8 of the manual for detail.
- Section 4.18.5 is added to manual, this section describes how to plot transition dipole moment contributed by molecular fragments as arrows in VMD program based on the data outputted by hole-electron analysis module of Multiwfn.
- In main function 1, if input "d", real space function value at a given point can be decomposed into contribution of various orbitals, see Section 3.3 for details. Property decomposition for critical points is also supported in option 7 of topology analysis module.
- MDL molfile (.mol) is supported as input file.
- Electronegativity Equalization Method (EEM) charge now can be easily calculated via suboption 17 of main function 7, see Section 3.9.15 for introduction and Section 4.7.5 for example. Atomic charges of a system composed of hundreds of atoms can be obtained instantly via this method.
- In main function 18, subfunction 8 is added to calculate amount of interfragment charge transfer between any two fragments during electronic excitation, see Section 3.21.8 for detail and 4.18.6 for example.
- Subfunction 22 of main function 100 is significantly extended, now it enables automatically detecting pi orbitals based on localized molecular orbitals for both planar and non-planar systems. This feature makes separate study of sigma and pi electrons quite easy for all kinds of system. See Section 3.100.22 of the manual for detail and Section 4.100.22 for illustrative application.
- Subfunction 9 is added to main function 9, this function can decompose Wiberg bond order in NAO basis as contributions from NAO orbital pairs and NAO shell pairs, and thus makes interactions between atomic orbitals and atomic shells that play key role of covalent bonding can be clearly revealed. See Section 3.11.8 of the manual for details and Section 4.9.4 for example.
- Main function 11 now can plot IR spectrum by using output file of Grimme's xtb program (https://www.chemie.uni-bonn.de/pctc/mulliken-center/software/xtb/xtb) as input file.
- Subfunction 16 is added to main function 200. This function is used to generate natural orbitals, spin natural orbitals and natural spin orbitals based on the density matrix in .fch/.fchk file. See Section 3.200.16 for details.
- Option 5 of subfunction 1 (hole-electron analysis module) of main function 18 now can output atom-atom contribution matrix of transition electric/magnetic dipole moment, and it can be further plotted as colored matrix map. See corresponding description in Section 3.21.1.3.
- Function -2 is added to main function 7 (population analysis module). Using this function the electrostatic interaction energy between two fragments can be calculated based on atomic charges. .chg file should be used as input file since this file records atomic charge information.
- The Strong Covalent Interaction index (SCI) proposed in DOI: 10.1021/acs.jpca.8b00521 is added as the 37th user-defined function, see entry 37 of Section 2.7 for introduction, this function is shown to be very useful for identifying very strong covalent bonds.

- Molden input file produced by Dalton is formally supported
- Mode 10 is added to the interface for setting up grid data, this mode allows the box to be defined in a GUI window, the position and size of the box can be visually defined and thus very convenient.
- Section 4.200.14.2 is added to the manual to illustrate how to use domain analysis module to visualize molecular cavity and calculate cavity volume.
- Section 4.19.2 is added to the manual to illustrate how to study variation of localized molecular orbitals (LMO) during chemical reaction. Section 4.19.3 is added to illustrate how to use LMO to analyze Re-Re quadruple bond in [Re2Cl8]2-.
- .molden file generated by ORCA with g angular moment basis functions now can be directly used as input file (without employing Molden2aim).
- "Set perspective" option is added to menu of all GUI windows for showing molecule, via this option one can exactly adjust viewpoint. In addition, in the GUI for showing relief map, text boxes are added to exactly control viewpoint.
- In Section 3.20.1 of Multiwfn manual, the way of plotting color-filled "RDG vs sign(lambda)rho" scatter map for studying weak interaction using NCI method is described.
- The functions 4,5,6 in hole-electron analysis module of Multiwfn is fully parallelized, the time cost is significantly lowered for large system.
- Orbital selection textbox is added to bottom-right corner of GUI of main function 0, so that orbital can be selected rapidly by simply inputting index. Beta orbitals can be selected by inputting negative index (e.g. inputting 7 and -9 means selecting the 7th alpha and the 9th beta orbitals, respectively).
- After booting up Multiwfn, if directly pressing ENTER button, a GUI window will be shown used to select input file.
- The built-in EDF library is updated (now it corresponds to molden2aim 4.1.4)
- Section 4.A.9 is added to manual. It describes how to calculate TrEsp (transition charge from electrostatic potential, see J. Phys. Chem. B, 110, 17268), and how to calculate excitonic coupling energy between two molecules based on TrEsp charges.
- When calculating ESP fitting charges (MK, CHELPG...), if radius of some elements are not predefined, now one can directly press ENTER button to use corresponding UFF radius multiplied by 1/1.2, this is often a reasonable choice.
- Speed of calculating transition dipole moment between excited state (subfunction 5 of main function 18) is greatly improved.

- For open-shell cases, the multi-center bond orders calculated based on NAO basis are not correct (prefactor is missing), this problem has been fixed.
- The sign of Mulliken transition charge outputted by hole-electron analysis module is not correct in the older versions, it should be multiplied by -1 to meet common convention. This problem has been fixed.

- The local total/dynamic/nondynamic electron correlation function proposed in J. Chem. Theory Comput., 13, 2705 (2017) now is supported as user-defined function 87,88,89, respectively. These functions are useful for vividly revealing electron correlation in various molecular regions. See corresponding entries in Section 2.7 for detail. Illustrative application is given in part 2 of Section 4.A.6.
- Spectrum of multiple systems now can be easily plotted together, see Section 4.11.6 for example.
- subfunction 2 of main function 100 now can yield basic input file for a batch of known quantum chemistry codes including Gaussian, GAMESS-US, ORCA, MOPAC, Dalton, MRCC, Molpro, NWChem, PSI, CFOUR and Molcas based on present geometry and charge/multiplicity.
- The CM5 charge proposed by Truhlar et al. in J. Chem. Theory Comput., 8, 527 (2012) has been supported as subfunction 16 of main function 7. See Section 3.9.14 of the manual for detail.
- The ghost-hunter index proposed by Adamo et al. in J. Comput. Chem., 38, 2151 (2017) is supported, it is automatically printed after hole-electron analysis is finished, see Section 3.21.7 of the manual for introduction. This index is useful to judge if an excited state calculated by TDDFT may be regarded as artificial ghost state.
- Gradient norm and Laplacian of electron density are added as user-defined function 79 and 80, respectively. The former is evaluated analytically, while the latter is evaluated semi-analytically.
- Electron delocalization range function EDR(r;d) and orbital overlap distance function D(r) are supported, the code is kindly contributed by Arshad Mehmood. Introduction is given as entry 20,21 of Section 2.6, illustrative examples can be found in Section 4.5.6, 4.5.7 and 4.12.8. Related references: J. Chem. Phys., 141, 144104 (2014); J. Chem. Theory Comput., 12, 3185 (2016); Angew. Chem. Int. Ed., 56, 6878 (2017).

- ORCA output file (CIS or TDA-DFT) has been formally supported for hole-electron analysis module, delta_r calculation module, NTO module and the module used to calculate transition dipole moment between excited states, see Section 3.21.1.2 for detail about the requirement on the input file. (TDDFT/TDHF output file may also be used, however, the result may be unreasonable when de-excitation is significant, see Section 3.21.1.2 for explanation)
- Output file of ORCA sTDA and sTD-DFT calculation now can be used as input file for plotting UV-Vis or ECD Spectrum via main function 11
- Better compatible with G16
- Anharmonic Raman and Anharmonic VCD spectra now can be plotted by main function 11 based on Gaussian output file of freq(raman,anharm) and freq(VCD,anharm) task, respectively (the latter is available only for G16)
- Main function -2 and -3 have been merged into main function 6 as subfunction -2 and -3.
- In subfunction 13 and 14 of main function 13, fragments can be directly defined by inputting atomic indices without preparing atomic list files. This change makes use of these functions more convenient.