Selecting modes¶

Mode Scanning, Mode Refinement and Mode Tracking as well as VG-FC Vibronic-Structure, VG-FC Vibronic-Structure Refinement and VG-FC resonance Raman all require a set of normal modes to operate on. For Mode Scanning these are the modes that you want to calculate the properties of, for Mode Refinement these modes form the basis modes, and for Mode Tracking these are the initial guess modes. For the VG-FC based methods these modes are the modes responsible for the vibronic coupling to the electronic excitation (in VG-FC Vibronic-Structure Refinement they are refined first).

Note

VG-FC Vibronic-Structure Tracking does not require any normal modes and as such does not support the ModeSelect (nor does it support the NormalModes block for that matter).

These methods provide options to load a large set of modes, after which the program will filter out the modes of interest. This is done according to the keys set in the ModeSelect block.

Note

The ModeSelect block is part of the NormalModes block of the Vibrational Analysis input. All Vibrational Analysis methods share this block, with the exception of VG-FC Vibronic-Structure Tracking. The methods for obtaining the set of modes that we will filter can differ per method. Particularly Mode Tracking features a lot of additional options, and the vibronic variants feature more specialized options.

Below is an overview of all the available options for the ModeSelect block as they appear in the basic vibrational analysis tools. The vibronic variants are discussed in more detail on their respective documentation pages.

The options below are not mutually exclusive, e.g.:

VibrationalAnalysis
NormalModes
ModeSelect
HighFreq 1
HighIR 1
End
... other options ...
End
End


This will select 2 modes: the one with the highest frequency and the one with the highest IR intensity. If these modes happen to be the same one however (the mode with the highest frequency also has the highest IR intensity), only 1 mode is selected.

ModeSelect

• HighFreq followed by an integer N will select the N modes with the highest frequencies.
• LowFreq followed by an integer N will select the N modes with the lowest frequencies. Imaginary modes are given with negative frequencies in AMS, and are included in this selection.
• LowFreqNoIm is the same as LowFreq except imaginary modes are omitted.
• ImFreq will select all imaginary modes.
• ModeNumber allows you to supply a list of integers. Each integer is the index of the mode in the order that they appear in the file. E.g. benzene has 30 vibrational modes, which are numbered 1-30.
• FreqRange selects all modes whose frequency falls in a specific range. 2 values must be supplied to mark this frequency range. Calculating all modes with e.g. frequencies higher than 3000cm-1 can be achieved by making the upper bound very large: FreqRange 3000 1000000
• IRRange selects all modes whose IR intensity falls in a specific range. 2 values must be supplied to mark this IR intensity range, the same way as for FreqRange.
• FreqAndIRRange combines FreqRange and IRRange. It selects modes in a frequency range whose IR intensity falls into a specified range as well. 4 values must be supplied: the first 2 specify the frequency range, the final 2 specify the IR intensity range.
• HighIR followed by an integer N will select the N modes with the highest IR intensities.
• LowIR followed by an integer N will select the N modes with the lowest IR intensities.
• Full requests a full frequency calculation. This will select all modes. This only make sense for Mode Scanning calculations, as tracking or refining all modes is just an overcomplicated way of doing the full vibrational analysis.

There are two options that are exclusive to the VG-FC based methods and which are not supported by Mode Scanning, Mode Tracking and Mode Refinement:

• LargestDisplacement followed by an integer N will select the N modes with largest dimensionless oscillator displacement in the VG-FC model. Requires an excited state gradient to be provided. See Vibronic-Structure for details.
• LargestBound selects all modes whose dimensionless oscillator displacement in the VG-FC model falls below the specified bound. Requires an excited state gradient to be provided. See Vibronic-Structure for details.
Type [ ModeScanning | ModeRefinement | ModeTracking | VibronicStructure | VibronicStructureRefinement | ResonanceRaman]

NormalModes

ModeSelect
# select the 2 modes with the highest frequency
HighFreq 2
# select the 2 modes with the lowest frequency (including imaginary modes)
LowFreq 2
# select the 3 modes with the lowest energy
LowFreqNoIm 3
# Select modes #1 #7 & #19
ModeNumber 1 7 19
# Select modes with frequencies between 3000 - 3200 cm-1
FreqRange 3000 3200
# Select modes with IR intensities between 5 - 10 km/mol
IRRange 5 10
# Select modes with frequencies between 1000 - 1500 cm-1 that have
# IR intensities between 10 - 30 km/mol
FreqAndIRRange 1000 15000 10 30
# Select the mode with the highest IR intensity
HighIR 1
# Select the 3 modes with the lowest IR intensities
LowIR 3
# Request all modes
Full true
# VG-FC only:
# Select the 7 modes with the largest VG-FC oscillator displacement
LargestDisplacement 7
# Select all modes with displacements larger than 0.01
DisplacementBound 0.01
End

End