So, you’ve optimized your molecule, obtained only real frequencies, and everything looks perfect—done, right? Well… not necessarily. For many well-behaved systems—especially typical organic molecules or main-group compounds with little to no radical character—a standard restricted or unrestricted calculation is sufficient. In such cases, the optimized wavefunction is usually reliable, and you can confidently extract molecular properties from it. However, there are situations where the computed wavefunction is not the most appropriate representation of the true electronic structure . This is where wavefunction stability analysis becomes essential. It allows you to test whether your solution remains stable when certain constraints are relaxed—for example, letting a restricted wavefunction become unrestricted or allowing orbitals to become complex. Importantly, even vibrational analyses are only strictly valid if the underlying wavefunction is stable. Instabilities are common...
Finding a transition state (TS) is often one of the most challenging tasks in computational chemistry . Success depends not only on choosing the right keywords in your route section, but also—crucially—on applying solid chemical intuition . Below are some practical strategies for locating TS structures using Gaussian . Method 1: QST2 Approach The first method you should attempt is the opt=QST2 keyword. In this approach, you provide the geometries of the reactants and products, and Gaussian uses the Quadratic Synchronous Transit (QST) algorithm to generate an initial TS guess, which is then optimized to a first-order saddle point . Example input: %chk=file.chk %nprocshared=n %mem=nGB #p b3lyp/6-31G(d,p) opt=qst2 geom=connectivity freq=noraman Title Charge Multiplicity Coordinates of reactants Charge Multiplicity Coordinates of products A critical requirement here is consistent atom numbering between reactants and products. Any mismatch will cause the calculation to fail. I...