Having solved the time-dependent Schrödinger equation both in real and imaginary time, we can move forward to investigate systems where the potential energy function has an explicit time dependence in it:

In this kind of systems, the expectation value of the Hamiltonian operator doesn’t have to stay constant.

Time-dependent perturbation theory is one method for finding approximate solutions for this kind of problems, but here I will handle a simpler example, which is called adiabatic approximation.

Suppose that the potential energy function is known. Now, let’s say that we also know the solutions of the time-independent Schrödinger equation

for any value of . I denote the solutions as , where it is understood that is a variable and is a parameter. Now, if the function changes very slowly as a function of time, i.e. its partial derivative with respect to is small at all points of the domain, we can use the adiabatic approximation, which says that if the initial state is the ground state for the potential , then the state at time is the ground state for the potential .

So, we can change a ground state of one potential into the ground state of another potential $V_2 (x)$ by making a continuous change from to slowly enough.

Let’s test this by chooosing a function as

i.e. a Hookean potential that moves to the positive x-direction with constant speed. If we set the wavefunction at to

which is the ground state corresponding to , the time depelopment of the wavepacket should be like

which means that is moves with the same constant speed as the bottom of the potential . This can be calculated with the R-Code below:

library(graphics) #load the graphics library needed for plotting lx <- 6.0 #length of the computational domain lt <- 15.0 #length of the simulation time interval nx <- 100 #number of discrete lattice points nt <- 300 #number of timesteps dx <- lx/nx #length of one discrete lattice cell dt <- lt/nt #length of timestep V = c(1:nx) #potential energies at discrete points for(j in c(1:nx)) { V[j] = as.complex(2*(j*dx-3)*(j*dx-3)) #harmonic potential } kappa1 = (1i)*dt/(2*dx*dx) #an element needed for the matrices kappa2 <- c(1:nx) #another element for(j in c(1:nx)) { kappa2[j] <- as.complex(kappa1*2*dx*dx*V[j]) } psi = as.complex(c(1:nx)) #array for the wave function values for(j in c(1:nx)) { psi[j] = as.complex(exp(-(j*dx-3)*(j*dx-3))) #Gaussian initial wavefunction } xaxis <- c(1:nx)*dx #the x values corresponding to the discrete lattice points A = matrix(nrow=nx,ncol=nx) #matrix for forward time evolution B = matrix(nrow=nx,ncol=nx) #matrix for backward time evolution for(j in c(1:nx)) { for(k in c(1:nx)) { A[j,k]=0 B[j,k]=0 if(j==k) { A[j,k] = 1 + 2*kappa1 + kappa2[j] B[j,k] = 1 - 2*kappa1 - kappa2[j] } if((j==k+1) || (j==k-1)) { A[j,k] = -kappa1 #off-diagonal elements B[j,k] = kappa1 } } } for (k in c(1:nt)) { #main time stepping loop for(j in c(1:nx)) { V[j] = as.complex(2*(j*dx-3-k*dt*0.05)*(j*dx-3-k*dt*0.05)) #time dependent potential } for(j in c(1:nx)) { kappa2[j] <- as.complex(kappa1*2*dx*dx*V[j]) } for(l in c(1:nx)) { for(m in c(1:nx)) { A[l,m]=0 B[l,m]=0 if(l==m) { A[l,m] = 1 + 2*kappa1 + kappa2[m] B[l,m] = 1 - 2*kappa1 - kappa2[m] } if((l==m+1) || (l==m-1)) { A[l,m] = -kappa1 B[l,m] = kappa1 } } } sol <- solve(A,B%*%psi) #solve the system of equations for (l in c(1:nx)) { psi[l] <- sol[l] } if(k %% 3 == 1) { #make plots of psi(x) on every third timestep jpeg(file = paste("plot_",k,".jpg",sep="")) plot(xaxis,abs(psi)^2,xlab="position (x)", ylab="Abs(Psi)^2",ylim=c(0,2)) title(paste("|psi(x,t)|^2 at t =",k*dt)) lines(xaxis, abs(psi)^2) lines(xaxis, V) dev.off() } }

and in the following sequences of images you see that the approximation is quite good for

By doing the same calculation again, but this time with , the image sequence looks like this:

where it is obvious that the approximation doesn’t work anymore.