Dr. Gaurav Jhaa Blogs

  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...

  2025 was declared by UNESCO as the International Year of Quantum Sciences , sparking widespread celebration and renewed attention toward quantum research. Yet, much of this discourse continues to be dominated by physicists and quantum technologists, many of whom enthusiastically proclaim the dawn of a new “ quantum revolution .” Amid this excitement, however, quantum chemistry appears noticeably underrepresented—despite being the discipline that has arguably done the most to transform quantum mechanics into a practical and predictive science. At the heart of this imbalance lies a difference in perspective. Physicists often engage deeply with the philosophical foundations of quantum mechanics, debating interpretations such as Copenhagen versus Many Worlds in conferences and popular science discussions. Chemists, in contrast, have traditionally taken a quieter, more pragmatic route—focusing less on interpretation and more on application. Few fields are as fundamentally grounde...

 The theoretical evaluation of a reaction mechanism ultimately hinges on locating the correct transition states (TS). However, despite the sophistication of modern computational methods, there is no guarantee that these methods will converge to the desired TS. This is where chemical intuition becomes indispensable—guiding the proposal of plausible reaction pathways and initial guesses. A transition state corresponds to a critical point on the Potential Energy Surface (PES): it is a minimum along all coordinates except one, along which it is a maximum. In systems with more than two degrees of freedom, the PES becomes a multidimensional hypersurface, making the visualization of a TS inherently challenging. Even in a three-dimensional representation (two degrees of freedom), the TS appears as a saddle point—analogous to the shape of a hyperboloid of revolution. A simple real-world analogy is a Pringles chip: at a given point, it curves upward in one direction (minimum) and downward ...