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New features
• Canonical Molecular Orbital (CMO) Analysis.
The CMO module provides a capsule description of the NBO composition of each canonical molecular orbital (occupied and virtual) as well as the percentage bonding, nonbonding, or antibonding character of each MO.
• Natural Chemical Shielding (NCS) Analysis.
Within the G9X-based GIAO framework, the NCS keyword gives the localized NBO/NLMO-based analysis of diamagnetic and paramagnetic contributions to NMR chemical shielding tensors.
Using G9X-based finite-field perturbation techniques, the NJC keyword leads to localized NBO/NLMO analysis of Fermi-contact-type contributions to scalar J-couplings (NMR spin-spin couplings).
• 3-c, 4-e Hyperbond (3CHB) Search.
The 3CHB keyword leads to detection and tabulation of strongly interacting hyperbond triads, corresponding to strong
A: B-C <===> A-B :C
resonance interactions (Pimentel-Rundle 3-c, 4-e bonding); see Sec. B.16. Hyperbond triads, although relatively rare in main-group bonding, are found to be a ubiquitous motif in transition metal bonding.
• Other New Keywords.
The new FIXDM keyword corrects unphysical (negative or Pauli-violating) occupancies of an input density matrix that is corrupted by numerical errors or poorly converged perturbative corrections, allowing completion of many analysis tasks that were previously aborted with fatal numerical inconsistencies. New keywords are also provided to print eigenvectors of the overlap matrix (SEV), write a Spartan-style archive file (SPARTAN), print the density matrix in the PNAO basis (DMPNAO), or print the table of interhybrid valence angles around all skeletal atoms (BEND).
• Larger Systems.
The entire program has been configured and formatted to allow easy re-dimensioning up to 999 atoms and 9999 basis functions [default: 200 atoms, 2000 basis functions]; see discussion of MAXATM, MAXORB parameters. This leads to slight changes in output format, designed to preserve the resemblance to previous versions with standard 80-character format for convenient screen and page display.
• Natural Resonance Theory (NRT) Improvements.
NBO 5.0 includes numerous changes in the NRT module to improve the generation of initial Lewis-like structures for transition metals and remove other bugs and numerical problems of the original implementation [S.F. Feldgus et al. J. Comp. Chem. 21, 411 (2000)].
• Natural Localized Molecular Orbital (NLMO) Improvements.
The NLMO routines now handle cases of non-Lewis orbitals with higher occupancy than Lewis orbitals, which formerly led to a halt. With the additional help provided by the FIXDM keyword (see above), NLMOs can now be successfully determined for a large fraction of the cases that failed in previous NBO versions. For platforms of lower numerical precision, NLMO automatically weakens numerical thresholds until the program can proceed as best possible.
• STERIC Improvement.
NBO steric analysis has been extended to include higher-order coupling effects between steric interactions and non-Lewis delocalizations. The extension consists of systematically replacing NBOs by NLMOs (and PNBOs by PNLMOs), in order to automatically incorporate resonance effects associated with the weak delocalization tails of the NLMOs. This leads to practically no numerical change when the system is well localized (e.g., rare gas interactions), but gives considerably improved description of steric effects in delocalized organic and organometallic species.
• New Checkpointing Options.
New checkpointing options are provided for open-shell calculations, allowing different checkpointed orbital lists for different spins. These options allow one to store specific localized PNBO electronic configurations (ground or excited) in the checkpoint file for post-SCF calculations or improved GUESS in SCF calculations.
• New Matrix Output Options.
NBO 5.0 now allows the user to select a small number of specific matrix elements for printing, rather than the entire matrix. Matrix elements may be selcted by numerical magnitude, by basis function serial number or descriptive label.
• Transition Metal Hybrid Directionality.
The default hybrid directionality and bond bending table now describes a much broader range of hybrid types, including the d-rich bonding hybrids of transition metals. In addition, the BEND keyword now prints out detailed tables of interhybrid valence bond angles for requested nuclei.
• Basis Linear Dependency Detection/Protection.
NBO 5.0 reduces the dimensionality of the NAO basis as necessary to remove linearly dependent components of the input AO basis set. The dimensional reduction is often consistent with measures taken by the host program (e.g., Gaussian or GAMESS), but NBO 5.0 detects and removes linear dependency to enhance numerical stability even if the host ESS does not.
• Other Fixes/Improvements.
The NBBP module has been corrected to avoid a condition that sometimes caused other keywords to be swallowed. The FEAOIN subroutine and keyword has been corrected to handle g-orbital input for correlation-consistent cc-pVnZ basis sets. The job-options control routine (JOBOPT) has been re-written to avoid compiler overflows on certain platforms. The $CHOOSE keylist implementation now allows more flexible specification of different Lewis structures for different spins in open-shell cases.
• New PC-Windows GENNBO Stand-Alone Version.
A fully functional stand-alone GENNBO implementation is now available (.EXE only) as GENNBO 5.0W for Windows 95/98/Me/NT/2K/XP systems. Just include the "ARCHIVE" keyword in a standard NBO-linked electronic structure job (even using older NBO versions!) to obtain the .47 archive file for input to GENNBO 5.0W. Carry out explorations of alternative NBO options from the convenience of your PC desktop, without recalculating the wavefunction.
Why NBO 5.0?
Relevance to Real-World Chemical Applications
NBO analysis provides the information you want: charges, bond types, hybrid directions, resonance weights, bond orders and other familiar valence descriptors. Why perform a complex ab initio calculation if you can't make chemical sense of the result?
Quintessentially Quantal
NBOs quantitatively extend, but are firmly rooted in, traditional orbital concepts of Mulliken, Pauling, and Coulson. Unlike methods based on numerically differentiating the charge density (a classical concept), NBO analysis remains closely tied to quantum mechanical wavefunction, phase, and superposition concepts.
A Chemist's Basis Set
Through their close association with localized Lewis-structure diagrams, NBOs provide a direct quantal link to familiar chemical valency and bonding concepts of Kekule, Werner, Lewis, and others. Unlike conventional MOs (whose sprawling delocalized forms can vary bewilderingly, even in closely related systems), the forms of NBOs are highly conserved, recognizable, and transferable from one molecule to another.
Established and Authoritative
NBO methods and terminology are increasingly recognized as standard in theoretical presentations, providing an established framework for state-of-the-art professional discourse. High numerical stability and rapid convergence insure maximum reliability and efficiency of the theoretical description. Widespread acceptance of the NBO paradigm by workers in all fields of chemistry is reflected in the burgeoning number of published applications (currently, >400 per year).
Broadest Range of Applicability
NBO provides a mutually consistent and comprehensive integrated set of analysis tools for energetic, dipolar, NMR, and other properties, insuring harmonious chemical interpretations from one property to another. Moreover, NBO is uniformly implemented across a wide variety of platforms, electronic structure packages, and methods (variational, perturbational, density functional), insuring highest possible comparitive consistency and utility.
Builds Enduring Bridges of Communication for Non-Specialists
NBOs give a simple and intuitive picture of the wavefunction, ideal for making theory meaningful and useful to beginning students of chemistry and affiliated sciences. Qualitative NBO-based concepts display increasing quantitative strength and generality as students progress to more advanced subjects.