Hello,
I am a user of CRYSTAL17 and would like to consult about some fundamental issues related to structural optimization and vibrational (frequency) calculations.
It is possible to calculate IR and Raman intensities and phonon DOS under various pressures using the "FREQCALC" block and specifying the "PRESSURE" (in MPa) after performing structural optimization. However, I am confused about the following:
Q1:
If structural optimization is performed using the default settings without specifying "EXTPRESS" (external pressure, in Hartree/Bohr³), the resulting structure corresponds to a (local or global) minimum on the potential energy surface at zero pressure. If a frequency calculation is then carried out with a non-zero "PRESSURE" applied, the potential energy surface effectively changes. This could result in imaginary vibrational modes, which is the source of my confusion.
Q2:
Similarly, if I perform a full structural optimization (including both cell parameters and atomic positions) under an applied external pressure (e.g., setting "EXTPRESS" to 0.001 Hartree/Bohr³ ≈ 29.457 GPa), I obtain a "contracted" structure with reduced cell parameters and shorter chemical bonds. If I then perform a frequency calculation on this optimized structure but set a "PRESSURE" value lower than the pressure used during optimization (e.g., less than 29.457 GPa), it seems reasonable to expect that imaginary modes might appear due to the mismatch between the pressure used in optimization and that used in the vibrational analysis. It is quite confusing.

Thanks~!